HONG KONG—China’s stock-market turmoil and deepening concerns about an economic slowdown are starting to hit the country’s vibrant technology sector, from the smallest ventures to multibillion-dollar startups.

Big Chinese startups such as Meituan.com, which has some similarities to group-discount website Groupon Inc., and social shopping site Mogujie are lowering the initially proposed valuations for their latest fundraising rounds. Meituan has cut its $15 billion asking price after discussions with investors, while private-equity firm Carlyle Group LP is no longer in talks to buy into Mogujie, according to people familiar with the deals. Some early-stage startups in the tech hub of Shenzhen are going out of business after failing to raise funds.

“Winter is coming…I think the startup boom is cooling down,” said Yu Xiaoyang, who left Chinese Internet company Tencent Holdings Ltd. last year to start an online medical consultation startup in Guangzhou. Mr. Yu, who is struggling to find new investors, said he is debating whether to return to a large company and take a more stable job.

The change in mood throws a cloud over one of the few bright spots in China’s economy: a flood of money and talent into tech firms and startups. The surge helped catapult companies such as Alibaba Group Holding Ltd. and Tencent into the ranks of the global Internet giants. In the past few years, more than a dozen Chinese startups achieved valuations of at least $1 billion.

Now, investors are wary of writing big checks to startups. A possible slowdown in consumer demand for tech products and services, as well as losses on stock investments, are affecting price tags for big startups and drying up cash for smaller deals, investors say.