The fundraising by Abogen is in its final stages, although the sum may change, according to two people who declined to be named because the information is not public yet.
It is not immediately clear how much SoftBank plans to invest, but a third person said the Japanese investment firm plans to lead the latest funding round.
The Suzhou-based firm, which raised more than $700 million in August from investors including Singapore's state investor Temasek Holdings and Hillhouse-backed GL Ventures, targeted a valuation of about 20 billion yuan ($3.1 billion) prior to the current fundraising, one of the people said.
Abogen is jointly developing a COVID-19 vaccine using mRNA technology with a research institute affiliated with the Chinese military and Walvax Biotechnology.
The potential vaccine has obtained approvals to begin Phase III clinical trials in Nepal, Mexico and Indonesia, according to Walvax, and is in the most advanced stage of clinical development among mRNA vaccine candidates that China is researching.
The candidate, named ARCoV, is yet to release peer-reviewed data from early-stage trials but it has received regulatory clearance in China to be tested as a booster dose for those who are inoculated with vaccines based on different technologies.
SoftBank declined to comment. Abogen did not immediately reply to Reuters' requests for comment.
($1 = 6.3888 Chinese yuan renminbi)
(Reporting by Julie Zhu, Kane Wu and Roxanne Liu)