The Briton returned to complete his third round before closing with a two-over-par 74 for a winning total of 16-under-par 272 in the weather-hit tournament.
Former Asian number one Thaworn Wiratchant finished two strokes adrift after carding 67 while the Indian duo of Gaganjeet Bhullar and Shiv Kapur shared third place a further two shots behind.
"Today was hard. It is never easy playing in these conditions and I had to play 32 holes," said Westwood who went into the final round with an eight-shot lead.
"It was draining out there and I was really feeling it on the back nine. When you are in contention and under pressure, it seems to zap your energy faster."
"You never know how to play with such a big lead. You don't know if you should attack or defend. It is quite hard to get your mental attitude around it.
"Hopefully this will kick-start my year. I've played well without any wins so hopefully I can go on and win other tournaments," added Westwood who finished tied for third at the U.S. Masters Tournament earlier this month.