A welding robot has a fifth arm. The fifth arm can rotate around a sixth rotation shaft and is rotatingly connected to the main body of a fourth arm. The shell of the fifth arm is generally in a cylinder shape. A through hole is formed in the shell, and the cross section of the through hole is in a round shape, and the central axis of the through hole is a seventh rotation shaft. The end face of the shell is provided with a first opening part. The opening edge of the first opening part in the shell is provided with an opening cover member, which is basically in a ring shape. The roughness of the opening cover member is less than the roughness of the outer surface of the shell.