The medium and small tonnage truck crane section size is small,so it is not conducive to assembly of twostage telescopic oil cylinder and pulley assembly telescopic mechanism in the boom barrel. In order to solve this problem, a twostage telescopic boom oil cylinder with small diameter, easy production, easy assembly and low cost was designed. Through trial production and test, it is verified that the oil cylinder can meet the operating requirements. The designed product provides reference for the design of telescopic mechanism of related hoisting products, and the product has been applied for Chinese patent.