official Xinhua news agency reported, citing maritime authorities.
They were caught in rough waters whipped up by Typhoon Wutip, which is expected to make landfall in Vietnam's central region later on Monday.
The boats, which were together carrying 88 fishermen, sank after they attempted to navigate gales near the Paracel Islands, about 330 kilometres (200 miles) from China's southern coast.
Altogether five boats carrying 171 people were caught in the stormy seas.
As of Monday around noon, rescuers had retrieved 14 survivors. A large-scale search and rescue effort has been mounted to find others, said Hainan Net, a website run by the government of China's southernmost province.
Ten ships, including three navy warships and six aircraft have been deployed for the rescue, the report said.
China's President Xi Jinping called for an 'all-out rescue effort', urging local authorities to 'do their utmost to find the missing or stranded and minimize casualties', Xinhua said.
But it added that rescue efforts have been hampered by strong gales and high waves.
The boats had come from the southern province of Guangdong.
China's maritime authority on Sunday raised its alert on the typhoon to orange, its second-highest of four levels, Xinhua reported.
Storm tides might strike the provinces of Guangdong, Hainan and Guangxi, it added, citing the National Marine Environmental Forecasting Centre.
Vietnam has so far evacuated 70,000 people from high-risk areas as it prepares for the typhoon, according to that country's national flood and storm control department.
"The typhoon is scheduled to make landfall at about 16:00 local time (1000 GMT) today in the central provinces - between Ha Tinh and Quang Tri," it said in a statement posted online.
Schools have been closed and all vessels along the central coastline have been ordered ashore, it added.
Xinhua earlier reported the number of missing at 75, though a later report said 74 people.
Updated figures could not be found on the websites of China's State Oceanic Administration or local maritime safety authority.
The Paracel islands are claimed by China, Vietnam and Taiwan, but Chinese fishermen often travel to the islands in search of bigger hauls.
A separate Typhoon Usagi last week left at least 25 people dead after crashing ashore in southern China, throwing transport systems into chaos and leaving tens of thousands of airline passengers stranded in Hong Kong.
That typhoon had prompted the highest-level alert from China's National Meteorological Centre.
The deaths occurred in Guangdong after the typhoon brought down trees and damaged roads. Dozens more were injured in accidents, state media said.
China's civil affairs ministry said 226,000 people were relocated due to that storm and more than 7,100 houses collapsed or were badly damaged.