The 23-year-old South African, who won two European Tour events on native soil at the start of the year, carded four birdies against one bogey to triumph at the Binhai Lake Golf Club in Tianjin.
Belgium's Nicolas Colsaerts was second after matching Grace's last-day round, while Englishman Richard Finch was third, a further shot back.
Grace, who won the Volvo Golf Champions and the Joburg Open earlier this year, came through the Tour qualifying school in the winter.
He was wayward off the first tee but recovered with a 15-foot par putt, while both he and Colsaerts birdied the second.
Grace bogeyed the par-four fourth, but he responded with birdies at the long sixth and struck his approach to five feet for another gain at the seventh.
Colsaerts birdied the seventh and ninth to move within two shots of Grace, but then bogeyed the 10th and saw his chances fade further with a Grace birdie at 12.
The South African parred the final six holes, while Colsaerts' birdie at the last allowed him to finish in outright second ahead of Finch.
The 34-year-old from Manchester moved up nine places to third with his six-under-par 66.
Grace told www.europeantour.com after his victory: "I stayed really calm today [Sunday].
"I kept hitting the ball well. I missed a couple of shots the first couple of holes but regrouped well and made the birdies where it counts on the par fives, and just stayed with it from there.
"I thought my first win was around the corner (this season). To have three before May is literally unbelievable, but I've been playing well so I've been feeling like I can win and I've been giving myself opportunities."
"I'm definitely here to win every week I play now."