The 54-hole tournament was forced into a weekday finish after torrential rain wreaked havoc in Hainan Island, delaying the final groups.
But Pace displayed no nerves to tap in for her first title on the women's tour, birdieing the 18th to finish at 16 under overall - and complete a five-under-par 67 final round.
Germany's Caroline Masson (67) was second at 13 under, while American pair Michelle Wie (70) and Jessica Korda (72) were a further two strokes back in a tie for third.
Masson claimed outright second with a hat-trick of birdies to finish her week, although left her shy of breaking her LPGA Tour duck.
American Danielle Kang (70) and South Korea's Chella Choi (72) were joint fifth at 10 under.