The Australian Open runner-up, who became the first Chinese player to reach a Grand Slam final this year in Melbourne, was able to chase down nearly everything Sharapova had to offer on Court Philippe Chatrier.
In Saturday's final, Li will face defending champion Francesca Schiavone. The fifth-seeded Schiavone, who last year became the first Italian woman to win a Grand Slam singles title, defeated No. 11 Marion Bartoli of France 6-3, 6-3.
The wind was a factor yet again at Roland Garros, and Sharapova's serve suffered because of it. The seventh-seeded Russian had 10 double-faults in the match, including on match point. Sharapova was attempting to become only the 10th woman to complete a career Grand Slam.
"She has a huge and big serve, so I was like: 'Please, double-fault. That way I can win the match,"' Li said. "I never believe myself I can be in final in French Open. I wish I can do even better on Saturday."
Li finished with 24 winners, twice as many as Sharapova, and saved eight of 11 break points.
In the final, Li will face either defending champion Francesca Schiavone or No. 11 Marion Bartoli. They were to play later Thursday in the other semifinal.
Sharapova was broken early in the first set, and then in her final two service games. In the eighth game, shortly after breaking Li to get back on serve at 4-3, Sharapova had three double-faults.
"I had some chances in the first set on her serve, a couple of games that went deuce and had a couple of break points," Sharapova said. "I felt like I had a short ball and just didn't step in and then just made an unforced error.
"Then you kind of give your opponent confidence ... and then at times I didn't serve well, and was rushing more than maybe I had to, and maybe went for -- considering the conditions -- maybe I was just trying to go for too big of second serves, especially."
The Russian broke Li to open the second set. But Sharapova's seventh double-fault gave Li the eighth game, making it 4-4.
"It's tough for me. It's tough for her," Sharapova said of the wind. "It's tough for both of us."
After reaching the semifinals on Wednesday, Li said she was surprised to go that far at the only clay-court Grand Slam. She has never won a title on clay, but she reached the semifinals at French Open warm-up tournaments in Madrid and Rome.
Against Schiavone in the final, she will face a woman who is more suited to the surface and who became the first player over 30 to reach a major final since Mary Pierce at the 2005 U.S. Open.
Schiavone won the final four games of her match and finished off Bartoli by breaking her at love. She then rubbed her hand into the red clay on center court before kissing it.
Last year, Schiavone dropped to the ground to kiss the clay after winning her first major title.