The move is part of efforts by China's government to rejuvenate state behemoths with private capital. Beijing added China Unicom last year to a first batch of state-owned enterprises to see mixed-ownership reform.
Reuters reported last month that Alibaba Group Holdings and Tencent would be among new investors putting a total of about $10 billion into China United Network Communications Ltd, China Unicom's Shanghai-listed unit.
With the addition of money from Baidu, China's biggest internet search provider, and second-largest e-commerce company JD.com, the total investment into that unit is likely to rise to about 80 billion yuan ($11.8 billion), the people said.
Baidu would invest about 10 billion yuan ($1.48 billion) and JD.com about 5 billion yuan, respectively, one of the people said.
Both sources declined to be identified as the talks are not public.
Baidu and JD.com declined to comment, and China Unicom, Alibaba and Tencent didn't respond to Reuters requests for comment.
China's State-owned Assets Supervision and Administration Commission (SASAC), which oversees state enterprises, also did not respond to requests for comment.
(Reporting by Julie Zhu in Hong Kong, with additional reporting by Cate Cadell in Beijing and Sijia Jiang in Hong Kong; Editing by Ian Geoghegan)
By Julie Zhu