To detect the three-dimensional distribution of underground pipeline,the laser spot variation on imaging screen is collected by camera. The method is of higher precision and more sensitive response. Five fixed points at the edge of the imaging screen are designed as the key points of scale calibration. The position of the five projected light ellipse and the laser light spot is obtained by collecting the images of the laser projection screen and the image process method. The data is more stable and reliable,compared with the previous method which detects the position of the laser spot related to the entire image. The technology improves the detecting accuracy of the three-dimensional detective instrument of the underground pipeline.