Why does Sanya use Hong Kong dollars?

Sanya uses RMB, not Hong Kong dollars.

Sanya is located in Hainan Province, China, and belongs to a city in China. Its currency is RMB, just like other places in China. Although Sanya is an international tourist city, the RMB is mainly used as the currency for local businesses and transactions. Sometimes it may happen that some hotels or businesses accept payment in foreign currencies such as Hong Kong dollars or US dollars, but this does not mean that Sanya uses Hong Kong dollars.

Hong Kong dollar is the legal tender of the Hong Kong Special Administrative Region, which is widely circulated and used in Hong Kong. Because Sanya is close to Hong Kong, as an international tourist city, it attracts tourists from all over the world, so Hong Kong dollars may also be found in some tourist places or shops, but this does not mean that Sanya uses Hong Kong dollars.