The method of point cloud reconstruction based on slicing technology needs to extract boundary points from slicing planar points. In order to solve the problem of extracting boundary points and overcome the drawback of low efficiency and bad result of current algorithms, a boundary points extraction method of planar point cloud based on multi-threshold was proposed. In the algorithm, k adjacent points were selected from the judged points, then the angle between the nearest points were calculated and the maximum angle was limited because there existed the biggest angle, thus the boundary points could be rapidly extracted. By analyzing the value of multi-threshold, testing the method to extract boundary points of different point cloud and comparing the proposed method with other three methods, the method accurately and better extracted boundary points regardless of the shapes. The experimental results show that the proposed method can well extract the boundary points on the condition of guaranteeing the original characteristic information and improves the speed and efficiency of point cloud reconstruction.