Aiming at the problem that the traditional line structured light plane calibration measurement accuracy is not high and the range of applications is small, a light knife plane calibration method of structured light system based on plane target was proposed. The iterative camera calibration was performed for target image without laser, and the light knife plane calibration was performed for laser target picture. The flat top laser detection center algorithm was proposed that made the light intensity coincide with the uniform distribution. The flat top laser was modeled as a rectangular step function, then the brightness of the background and foreground brightness were estimated to determine light stripe width, and then the light stripe centerge was obtained by calculating the center of gravity within the window of effective pixels. The algorithm was used to deal with pictures with different noise and different blocks, the results show that the root mean square errors are within 0.149 pixel and 0.176 pixel, respectively, which means that the algorithm has high anti-noise ability and high precision. The light strip center was extracted by using the proposed algorithm, the position of the light bar on the target was calculated. According to the light strip center 3D points of at least two postures, the light knife plane was fitted by least squares. By means of iterative camera calibration and knife plane calibration, 3D point cloud data was obtained in the stereo vision model by using triangulation. Two standard balls with mutual distance of 100.5mm were experimental measured, the distance between the camera and the standard balls is 500mm, the distance between two ball centers and the standard distance were compared, the measured average error is 0.236mm. It shows that the flat top laser detection center algorithm is practicable, and the method of light knife plane calibration basically meets the requirements.