In the other Group B game, Malaysia's hopes of progressing suffered a big blow after a horrendous goalkeeping blunder two minutes from time gifted Vietnam a 3-2 win.
Thailand lead the group on six points, followed by Vietnam and Malaysia on three, with Laos bottom after two defeats. Thailand face Malaysia and Laos take on Vietnam in Wednesday's remaining group matches.
Thailand coach Peter Reid opted to field a second-string team against the part-timers. They went ahead on 19 minutes through Ronnachai Rangsiyo and Patiparn Phetphun hit their second 11 minutes later.
Arthit Soontornpit's sweetly-struck freekick sailed into the top corner to make it three on 40 minutes. He scored again soon after halftime with a curling freekick and Anon Sangsanoi completed the rout with two goals in the last 11 minutes.
"They did a smashing job, they did brilliant, this was our B team so it's made selection difficult for me," Reid told Reuters.
Pham Luong fired Vietnam ahead against Malaysia on 16 minutes, Indra Putra Mahayuddin equalised moments later before Nguyen Vu Phong restored the lead on 72 minutes.
Vietnam gave away the advantage near the end, when keeper Duong Hong Son was penalised for holding the ball too long, with Mahayuddin equalising from the resultant indirect free kick.
Malaysia were condemned to defeat, though, when Phong's long ball forward from the back bounced over goalkeeper Mohamed Elias and into the net.
"We've a very slim chance of reaching the semi-finals now and it's down to three mistakes by our keeper," Malaysia coach B. Sathianathan told Reuters.
"We were the better team, but this is football."
