In the narrow and dangerous working environment, the welding robot is an important way to guarantee the welding quality and improve the welding efficiency. During the process of the welding, the robotic abilities of environment perception and 3D measurement are the premise of robotic automatic planning and automatic control, especially in the narrow environment. Because the laser sensor has the characteristics of compact structure, non-contact measurement and high precision, the laser sensor is used to design a new 3D measurement system of welding workpiece combined with the practical application requirements of aluminum electrolytic cell. This system combines off-line programming mode of the special welding robot in the virtual environment to achieve semi-automatic batch welding tasks. At the same time, a calibration method based on monocular vision is designed to calibrate the laser sensor. Through the experimental verification, the system can well realize the 3D measurement of welding workpiece. Meanwhile, the angle measurement error is lower than 0.5 degree. And the results meet the actual welding demand through the actual welding experiment verification.