Baidu, Ctrip's biggest shareholder with a 19% stake, is offering 31.3 million American depositary shares (ADS), each representing 0.125 ordinary share of Ctrip. Underwriters will have a greenshoe option of 4.7 million ADS, Ctrip said in a statement.

U.S.-listed shares of Baidu were up nearly 3% at $105.96, while Ctrip dropped nearly 4% to $31.02 in early trading on Thursday.

Baidu and Ctrip in 2015 agreed to a share swap deal in which Ctrip exchanged some of its newly issued shares for stock that Baidu held in rival travel firm Qunar Cayman Islands.

Baidu, whose search engine dominates the market in China, is struggling amid tougher government regulation and as a trade war-hit economy has sapped customers' wallets, hurting advertising revenue.

While Baidu has been expanding into other business lines such as cloud services and mini programmes within its Baidu App, most of its success so far has been at video streaming service iQiyi which competes with Alibaba-backed Youku and Tencent Holdings' Tencent Video.

Recently Baidu invested in science forum Guoke, and Zhihu, China's answer to Quora.

"It's a strategic move for Baidu," said Xue Yu, tech analyst at market research firm IDC.

Baidu is pulling away from businesses like Ctrip's that connect online consumers to offline services like hotels, and focusing on building its own content ecosystem, Xue said.

Ctrip owns Scottish travel website Skyscanner and U.S. travel research site Trip.com.

Baidu has lost more than a third of its market value this year. Ctrip stock, which closed at $32.15 on Wednesday, has risen 18%, but has slipped in the past quarter.

Goldman Sachs Asia LLC and J.P. Morgan Securities LLC are acting as the joint book-runners for the proposed offering, Ctrip said.

Baidu declined to comment.

(Reporting by Yingzhi Yang in Shanghai and Sayantani Ghosh in Singapore; additonal reporting by Ambhini Aishwarya; editing by Jason Neely and Anil D'Silva)