The Pinandŏk Mountains pass through northern and eastern Tongchang, contributing the county's highest point, Tanphungdŏksan (단풍덕산, 1159 m).

The Changsŏng River flows from the northwest to the southeast, on its way to join the Taedong.

Some 80% of the county's land is forested (of which 40% is coniferous, with mixed pine-oak woods predominating).

Tongchang county is divided into 1 ŭp (town), 1 rodongjagu (workers' district) and 16 ri (villages): The year-round local temperature is 7 °C, with an average of -10 °C in January and 22.8 °C in August.

Tongchang has the least cultivable land of any county in the province, at 6.5% of its total area; most of that (80%) consists of dry fields unsuitable for rice.