Aiming at the problems such as narrow operating space,low manual welding efficiency and unstable quality when laying urban water supply pipeline networks，a modular pipeline welding robot was designed.Using modular design method,the mechanical structure design of the welding robot was completed which mainly included three parts:the welding walking chassis,the torch attitude adjustment mechanism and the adjustable flexible guide rail.The kinematic equations of the robot were established using the D-H parameter method,and the visualized workspace of the robot was simulated and analyzed based on Monte Carlo method.Finally,the modular pipeline automatic welding process experiment and construction site application were carried out.The experimental results and construction site application results show that the robot can complete the all-position welding task of the corresponding wall thickness steel pipe with stable welding process and good weld seam formation.