Teerasil Dangda scored the first goal for the War Elephants at Thammasat University Rangsit Campus in Bangkok, slotting home a penalty at the end of the first half.
Sumanya Purisai bagged the second goal for the hosts five minutes into the second half, before Chanthea Sieng grabbed a goal back for Cambodia in the 68th minute.
Teerasil secured victory for Thailand with his second goal in the last minute of the game.
Thailand finished top of the group on goal difference from Indonesia, both teams on 10 points. Cambodia were third on six points, followed by the Philippines (three) and Brunei (no points).
Photographer : Wanchai Kraisornkhajit
Thailand will face the Group B runners-up in the two-leg semi-final, in a repeat of last year’s final. The away leg is on January 7 while the home leg in Bangkok is on January 10. All Thailand matches are broadcast live on MCOT HD30 and T-Sport 7 channels.