Abstract: A positioning errors equation of airborne LIDAR point clouds was founded based on its positioning equation. Errors of point clouds were classified into three categories-systematic errors, mission errors and random errors-according to LIDAR positioning equation. The influences of slope and scanning angle for positioning errors of point clouds were analyzed in detail under planar surface, down-sloping surface and up-sloping surface conditions. The influences of scanning angle errors, mounting errors, attitude errors as well as scanning angle for positioning errors of point clouds were discussed. Except of analyzing systematic errors, influences of time deviations, GPS positioning errors, lever-arms errors and random errors for positioning errors of point clouds were also emphasized and analyzed. The results of research showed that flight height and scanning angle were crucial error sources, down-sloping surface and scanning angle errors in all error sources had great impacts on positioning errors of point clouds, mounting errors could be eliminated by calibration method, attitude errors were only rely on IMU's accuracy alone.