More women, and some men, have found themselves caught up in similar disputes. Divorce has become increasingly common in China, and as the middle class and private businesses have expanded, growing numbers of divorces involve debts that must be divided up, along with children, pets and homes.

China’s current marriage law dates to 1981, a time when divorces and loan disputes were much rarer. Though it has been amended over the years, it still leaves much unclear about how debt should be divided up.

In 2003, the nation’s top court, the Supreme People’s Court, tried to offer certainty by telling judges to presume that both spouses in a divorce shared liability for personal debts. That rule helped prevent the use of sham divorces to escape debts. But it left a burden on divorced spouses disputing debts to prove that a loan agreement had clearly excluded them from liability.

For Ms. Li and many other divorced women, that attempt at legal clarity has simply created more confusion about how to prove they had no responsibility for a debt.

“As long as a debt was acquired during marriage, the presumption was that it was shared,” said Jiang Yue, a law professor at Xiamen University in southeast China. A former husband or wife had to prove that a debt was not jointly held, she said. “But if they never knew about it to begin with, how could they assume the burden of proof?”

Ms. Li’s journey to activism began in 2014, after she and her husband, Li Xianghua, divorced. Their marriage had faltered while he was consumed with his automobile parts business and she worked as a journalist at the Farmers Daily, an official newspaper for agricultural news, she said.