Authorities cancelled all flights and trains from Sanya, known as "China's Hawaii", on Saturday, a day after 263 positive cases were confirmed.
Travellers must now present five negative PCR tests over seven days before being allowed to leave.
China is the only major economy to still follow a "zero Covid" policy.
The country has recorded fewer than 15,000 deaths since the pandemic began, according to Johns Hopkins University. However, there have been concerns about the impact of the strict rules, including mass testing and local lockdowns, on the economy.