Abstract: A small mobile welding robot was designed to solve the problem during automatic welding in narrow space. According to motion characteristics of robot structure, the welding robot was divided into three parts by using the modular design method. And the differential drive method was used in three wheeled mobile platform which had an adjustable magnetic force device at the bottom, and the controller was integrated in the platform. The welding gun regulating mechanism was composed of a cross slider and a welding gun connecting device, and the cross slider was driven by two motors respectively which could track welding seam in level and height directions. A conical pendulum form was adopted in the arc sensor, and it was integrated with the welding gun for easy to use. At the same time, the kinematics equation was built, and the different turning centers for a right-angle fillet welding were also analyzed to determine the parameters of horizontal slider distance. This will provide references for the control of welding robot. The experimental results show that the design requirements for tracking curved seam were satisfied and good welding quality was acquired.