Teerasil Dangda headed home the first goal from a free kick in the third minute at Thammasat University Rangsit Campus before bagging his second with a penalty at the end of the first half.
Adisak Kraisorn made it 3-0 in the 57th minute and Suphanan Bureerat scored another six minutes later.
Thailand have won both of their opening group A games and will play Indonesia on Thursday (December 29).
The War Elephants lead the group on goal difference ahead of Indonesia, with both teams on six points. Cambodia are third with three points, level with the Philippines who have played one game more.
Brunei were stomped 7-0 by Indonesia and remain bottom of the group with no points from three matches.