This paper introduces a positioning method based on the robot arm and laser-camera method, which is mainly oriented to the on-site additive construction and blockwork using mobile robots, and ensures the positioning accuracy of about 5 mm. This method provides a location scheme that is cheaper than directly using total stations and has higher accuracy than LiDAR or VSLAM. In addition, the method overcomes the limitation of the arm span on the positioning process: it is usually located by scanning the ArUco marker on the site by the camera at the end of the robot arm, which means that intensive positioning markers are required. The main equipment involved in the positioning process includes a laser light mounted at the end of the robot and a laser receiving panel as a marking point. The receiving panel is observed by an RGB camera to ensure that the light spot is projected in the center of the panel. The whole process is as follows: in advance, a total station is used to arrange several receiving panels at an interval of 5 to 10 m on the site, and then the mobile robot will shoot the laser at nearby receiving panels in turn, and push back its position through each axis angle to complete the positioning. The accuracy of this method mainly depends on the accuracy of the 4–6 axis of the robot. Therefore, this paper also introduces a calibration method to measure the mechanical error of the robot arm itself, to improve the positioning accuracy as much as possible. With the help of this method, many construction processes will achieve mobile construction and can bring some innovation to the construction process.