Until 1972, North Korea's official capital was Seoul, South Korea because they always planned to retake the south.

Pyongyang wasn’t officially considered the capital of North Korea for several decades. The regime in the north was disillusioned with their power and refused to accept that South Korea wasn’t in their powerful grip any longer. They positioned themselves as the sole legitimate government of the entire Korean peninsula. 

It wasn’t until 1972 that they dropped Seoul from their constitution and made Pyongyang the capital. Apparently, the way the North Korean government saw the situation was that Seoul and the south was under American occupation and their South Korean client. 

Due to the “occupation,” Pyongyang was considered the provisional headquarters of the peninsula’s sole government. It was to be used only until the eventual “liberation” of Seoul. Finally in 1972, Pyongyang was officially made the national capital. Pyongyang is located in west-central North Korea on a flat plain east of the Korean Bay. The Taedong River flows through the city. The name Pyongyang literally means “flat land.” It is the largest city in the country. 

(Source)