The county is primarily mountainous, and is traversed by the Masingryŏng and Kwangju ranges.

The chief streams include the Namdaech'ŏn, Yongjich'ŏn, and Komitanch'ŏn (고미탄천).

Sep'o county is divided into 1 ŭp (town), 1 rodongjagu (workers' district) and 24 ri (villages): Sep'o is host to deposits of molybdenum, silver, zinc, and fluorite.

Agriculture also contributes to the local economy; Sepo is particularly known for its radishes.

In addition, livestock raising and orcharding play a role, and there is some small-scale manufacturing as well.