On 16 April 2014, the AFC Executive Committee approved a proposal to merge the preliminary qualification rounds of the FIFA World Cup and the AFC Asian Cup, which expanded to 24 teams starting in 2019. Therefore, the first two rounds of the FIFA World Cup qualifiers also acted as qualifiers for the 2019 AFC Asian Cup in the United Arab Emirates.
First round: 12 teams (ranked 35–46) played home-and-away over two legs. The six winners advanced to the second round.
Second round: 40 teams (ranked 1–34 and six first round winners) were divided into eight groups of five teams to play home-and-away round-robin matches. The eight group winners and the four best group runners-up advanced to the third round of FIFA World Cup qualification as well as qualifying for the 2019 AFC Asian Cup finals.
Third round: The 12 teams (an increase from 10 for 2014) which had advanced from the second round were divided into two groups of six teams to play home-and-away round-robin matches. The top two teams of each group qualified for the 2018 FIFA World Cup, and the two third-placed teams advanced to the fourth round.
Fourth round: The two third-placed teams of each group from the third round played home-and-away over two legs. The winners advanced to the inter-confederation play-offs.
The 46 FIFA-affiliated nations from the AFC entered qualification. In order to determine which nations would compete in the first round and which nations would receive a bye through to the second round, the FIFA World Rankings of January 2015 were used (shown in parentheses), as those were the latest published rankings prior to the first round draw. The FIFA Rankings of January 2015 were also used for seeding of the first round draw; however, for seeding in the second round and third round draws, the most recent FIFA Rankings prior to those draws were used.
^ abcTimor-Leste won the first leg 4–1 and the second leg 1–0, thus winning 5–1 on aggregate and advancing to the Second Round. On 12 December 2017 FIFA awarded both matches 3–0 to Mongolia due to Timor-Leste fielding numerous ineligible players. However, this was long after the Second Round had been played so Timor-Leste advanced and Mongolia were not reinstated.
^ abcdefTimor-Leste forfeited five group matches due to fielding numerous ineligible players.
^The Malaysia v Saudi Arabia match, on 8 September 2015, was abandoned during the 87th minute after a group of supporters threw objects onto the pitch. At the time of the abandonment the score was 2–1 to Saudi Arabia. On 5 October 2015, FIFA decided that this match was forfeited by Malaysia and the result to be declared as a 3–0 win awarded for Saudi Arabia.
^On 30 May 2015, FIFA announced that the Football Association of Indonesia (PSSI) was suspended with immediate effect for governmental interference. On 3 June 2015, the AFC confirmed that Indonesia have been excluded from the qualifying competition, and all matches involving them have been cancelled.
^ abcdOn 16 October 2015, the Kuwait FA was suspended by FIFA with immediate effect, after Kuwait had failed to comply with a decision from the FIFA Executive Committee that sports law of the country had to be changed before 15 October. The three remaining matches involving Kuwait (away v Myanmar, home v Laos, away v South Korea) were not played as originally scheduled, and were later awarded as 3–0 wins for Kuwait's opponents.
^FIFA awarded North Korea a 3–0 win as a result of Yemen fielding the ineligible player Mudir Al-Radaei, after North Korea had defeated Yemen 1–0. Al-Radaei failed to serve an automatic one match suspension for receiving two yellow cards earlier in the First Round of the competition.
As a result of Indonesia being disqualified due to FIFA suspension, Group F contained only four teams compared to five teams in all other groups. Therefore, the results against the fifth-placed team were not counted when determining the ranking of the runner-up teams.
The two third-placed teams in each group from the third round played against each other home-and-away over two legs to determine which team advanced to the inter-confederation play-offs.
The order of legs was announced during the draw for the third round. Syria played their home match, as with all their home matches in the third round, in Malaysia due to the war time condition in Syria.
The draw for the inter-confederation play-offs was held as part of the 2018 FIFA World Cup Preliminary Draw on 25 July 2015, starting 18:00 MSK (UTC+3), at the Konstantinovsky Palace in Strelna, Saint Petersburg. The fifth-placed team from AFC was drawn against the fourth-placed team from CONCACAF, with the AFC team hosting the second leg.