He finished at 19-under 265 to edge Taiwan's Lin Keng-chi (67), who entered the day tied for the lead with Ted Purdy of the United States. Purdy faded with a 74 and finished eight strokes behind to tie for seventh.
Purdy held the lead alone for the first two days, with rounds of 62 and 66. He had a 71 Saturday and his score Sunday was the worst among the top 25 finishers.
India's Jyoti Randhawa was third, shooting a 65 for a 269 total.
Thongchai arrived in Thailand from the United States one day before the tournament after an unsuccessful effort at the PGA Tour Q-School in Florida.
This was his fourth Asian PGA Tour victory. He was third this year on the Asian PGA Tour Order of Merit, after coming in second last year and winning the title in 2001.
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