The win sees the 38-year-old move above Northern Ireland's Rory McIlory into second in the world rankings.
The Englishman made a stunning start to the Asian Tour event and was a record 18 strokes under par after 36 holes.
And after his final-round 69, Westwood said: "I've amazed myself really. It's been staggering."
The Worksop golfer began the day only four shots clear of South African Schwartzel, having led by 11 at halfway after following up a stunning 12-under-par 60 in his first round with a 64 in his second.
The gap was reduced to three shots when Schwartzel birdied the opening hole of the final round, but Westwood quickly struck back with two birdies of his own at the second and third holes.
Masters champion Schwartzel had a glimmer of hope on the back nine when he eagled the par-five 11th to, once again, get within three. But Westwood struck back with three birdies between the 12th and 15th holes.
Schwartzel then bogeyed the penultimate hole and slipped further behind with a five at the last.
Westwood, who has rejoined the US Tour for 2012, said he plans to take a break over the Christmas period and then improve on his fitness for the new year.
He told Sky Sports: "I'm going to continue to work on my fitness and all aspects of my game. I'm going in the right direction, I can see a lot of improvement and I'm happy with the way I've finished the year, I've won two out of my last three events.
"I'll go into the winter with a lot of confidence and I'll try to come out next year playing the same kind of golf as I played this week, a little bit leaner and well rested."
England's Simon Dyson shot six birdies against three bogeys for a 69, sharing fourth place with Thailand's Chawalit Plaphol, who shot a 67 which was the equal best round of the day with Germany's Alex Cejka, who was eighth.
