The third-ranked English star finished at 22-under 266 at Amata Spring Country Club. He opened with rounds of 60 and 64 and shot a 73 on Saturday in the Asian Tour event.
"It means a lot, any win does," Westwood said. "It's special this week because of the difficulty of the golf course, who I was up against in the weekend.
"Charl is the Masters champion and he has had a great year. The fact that I led wire to wire — 60 in the fisrt round and 64 in the second round, 20 under after two rounds — it's yours to lose really with an 11-shot lead. There was pressured involved."
Schwartzel closed with a 72.
"It was a tough day. The wind probably blew the hardest today compared to the last three rounds," the South African star said. "I played great. If it wasn't for Lee, I would have probably won the tournament."
American Michael Thompson was another stroke back after a 70.