Money-market funds' total assets surged six-fold in the 18 months through December, rising to 2.2 trillion yuan ($351.3 billion), according to Fitch Ratings.

The expansion was driven mainly by retail investments in e-commerce related funds, Fitch analysts Li Huang, Charlotte Quiniou and Alastair Sewel wrote in a report.

Retail investors accounted for more than 70 percent by the end of the second half of last year, the report said, adding that the five largest asset managers held 51 percent of total assets.

Yu'ebao, managed by Tianhong Asset Management Co and sold online by Alibaba Group Holding Ltd, was the largest with 26 percent.