Sanmen County is located in the northeast coastal area of Taizhou City, Zhejiang Province. Tiantai Mountain in the west, Sanmen Bay in the east, Ninghai County in the north and Linhai City in the south. Northwest high terrain, low southeast. In the middle and west, there are low hills and hills, and in the east, there are coastal plains. The county covers an area of 1510 square kilometers, including nearly 500 square kilometers of sea area. It has jurisdiction over 6 towns, 1 township and 3 subdistricts. According to the seventh census, as of midnight on November 1, 2020, Sanmen County had a permanent population of 379,469.

In 1928, the Communist Party of China launched the peasant armed uprising in the Pavilion, the establishment of Zhejiang Province's first Soviet regime, liberation on February 17, 1949.
Sanmen County is one of the first pilot areas of Zhejiang high-quality development and construction of common prosperity demonstration zone, the national rural governance system construction pilot unit, the national health township (county).

In 2021, the GDP of Sanmen County reached 31.95 billion yuan, an increase of 8.6% over the previous year based on comparable prices. That of the primary industry was 3.663 billion yuan, that of the secondary industry 15.445 billion yuan and that of the tertiary industry 12.842 billion yuan, up 3.2 percent, 10.9 percent and 7.6 percent respectively. The structure of the added value of the tertiary industries has been adjusted from 12.1∶47.4∶40.5 in 2020 to 11.5∶48.3∶40.2. The per capita GDP was 84,190 yuan (based on the permanent population), an increase of 8.5%.