The 31-year-old Li, Asia's lone Grand Slam singles champion, overcame a stuttering start before completing her first successful title defence on the women's tour in an hour and 49 minutes.
Trailing 3-4 in a showdown between China's top two players, Li bagged seven of the next eight games to move 6-4 4-1 ahead before fifth-seeded Peng resisted, winning four in a row to lead 5-4 in the second set.
Former French Open champion Li responded by winning the next three games to claim her eighth WTA title while prolonging Peng's wait for a maiden crown in the sixth singles final of her career.
This was a special victory for Li, who featured in the first all-Chinese WTA final against Zheng Jie in 2006 when she was forced to retire through heat exhaustion at the Estoril Open clay court event in Portugal.
The Shenzen tournament acts as a warmup event for the Australian Open, the first grand slam of the season, which starts in Melbourne on Jan. 13.