Wu Song was from Qinghe Prefecture. He was good-looking, with eyes which shone like stars, thick eyebrows, a wide chest and a muscular body. He stood at 8 feet and gave people an impressive bearing. Once, he knocked out a person cold in a drunken fit. Misunderstanding that he had killed that person, he fled to Chai Jin's home for security and met Song Jiang there. The two later became sworn brothers.