Cheng County or Chengxian (simplified Chinese: 成县; traditional Chinese: 成縣; pinyin: Chéng Xiàn) is a county under the administration of Longnan City, in southeastern Gansu Province of China.

The postal code is 742500, and the county seat is located in Chengguan Town.

[2] In January 2021, the Qingni River in Chengxian was found to be polluted with thallium from zinc production wastewater.

Longnan Chengzhou Airport opened in 2018 and offers domestic flights.

China National Highway 567 and G7011 Shiyan–Tianshui Expressway are the main roads connecting the county.