Born in Hamhung City, South Hamgyong Province, Ku began to learn boxing at the age of 12.

When he was 17 years old, he joined the Amnokgang Sports Team and rose to fame as national champion in the 48 kg category.

His boxing techniques were fully demonstrated at the 4th International Boxing Tournament for Golden Belt in Juche 64 (1975). In the tournament, Ku defeated his Romanian rival, a European champion, and Turkish rival in the 54 kg category to win a golden belt and a gold medal.

At that time, a number of foreign media spoke highly of his boxing techniques.

He took the titles at the 5th and 6th international boxing tournaments for Golden Belt, thus becoming a thrice golden-belt winner.

He bagged a gold medal in the 54 kg category at the 21st Olympic Games. In the games, all of his matches came under spotlight of experts and spectators.

He also took the titles in the 7th and 8th Asian games, bringing the number of his gold medals to more than 10.