The county spans an area of 880 square kilometres (340 sq mi), and has a permanent population of 150,522 as of 2019.

[1] Fenxi literally means west (Chinese: 西; pinyin: xī) of the Fen River (Chinese: 汾; pinyin: fén), due to the county's location along the western banks of the Fen River.

[2] The Northern Wei reorganized the area was Fenxi Commandery (Chinese: 汾西郡; pinyin: Fénxī Jùn).

[2] Fenxi County is located along the west bank of the Fen River, and along the southeastern portion of the Lüliang Mountains.

[3] The county's terrain is higher in the northwest, lower in the southeast, and generally characteristic of the Loess Plateau[3] The county's highest point is Xigushe Mountain (Chinese: 西姑射山; pinyin: Xīgūshè Shān), at 1,890 metres (6,200 ft) above sea level; the county's lowest point is in southern Tuanbai Township [zh], at 583 metres (1,913 ft) above sea level.