The film tells the moving end-of-life story of Fang Xiuying, a farmer who has been struggling with the dementia-type disease for eight years.
After a stay at a nursing home, her family brings her home, and the film focuses on their final days together before her death in 2016.
Wang Bing counts among China's foremost documentary film makers, and has previously served on the Locarno jury.
Now in its 70th year, the Locarno festival takes place every summer in the picturesque Swiss town on the shores of Lake Maggiore.
The runner-up Special Jury Prize went to Brazilian horror film As Boas Maeiras (Good Manners) by directors Marco Dutra and Juliana Rojas, which starts off with a woman hiring a maid to help with the housework before taking an unexpected, blood-drenched turn.
France's F. J. Ossang meanwhile won Best Director for film noir 9 Fingers, the organizers said.
French star Isabelle Huppert was named Best Actress for her role as Mrs Hyde in French director Serge Bozon's film of the same name.
And Danish actor Elliott Crosset Hove took home the Best Actor award for his role in Icelandic director Hlynur Palmason's debut film Winter Brothers.