A portion of the county is occupied by the Ryongam wetland, which also crosses into Ch'ŏrwŏn. Most of the county's terrain is mountainous, although there a few small expanses of level ground. The Kwangju and Majŏllyŏng mountains pass through P'yŏnggang. The region is prone to heavy rains.
The P'yŏnggang area was known as Puyang hyŏn during the Koguryŏ period, and as Kangp'yŏng (강평) under Silla. In the Koryŏ Dynasty, it was included in Tongju (동주); in the Chosŏn dynasty, it took its modern name.