Li Changgeng was born in 1750 in Tong'an, Fujian, passed the military metropolitan examination in 1771, and spent the rest of his life in the navies of Fujian and Zhejiang.

In the early nineteenth century Cai Qian's fleet was collecting protection money and seizing merchant ships across the Taiwan Strait and along the Fujian and Zhejiang coast; the Qing navy's ships were smaller than the pirates' and carried fewer guns. Li's answer was to build first: the large warships he oversaw, called "thunder ships," were meant for pursuit on the open sea. Around 1804, as Admiral of Zhejiang, he was given command of the combined Fujian and Zhejiang fleets with the single task of destroying Cai Qian.

For the next few years he chased Cai from Zhejiang to the Taiwan Strait, cornering him more than once and losing him more than once. In 1806, when Cai attacked Lu'ermen and besieged the Taiwan prefectural city, Li brought the fleet across to relieve it.

In December 1807 he caught Cai's fleet in the Black Water off the Guangdong coast and was killed by cannon fire in the battle, aged fifty-seven. The court made him a third-class Earl Zhuanglie. Two years later his successor, Wang Delu, drove Cai Qian to blow up his own ship off Zhejiang.