The error model of four-prism based on the unique scan four-prism tower mirror of domestic airborne laser scanner is put forward. And the position error models in horizontal and vertical direction are established respectively. The effect of positioning accuracy in qualitative and quantitative aspects are analyzed. The analysis demonstrates that the influence of error in the vertical and horizontal direction is different, which is related to the scanning angle and increases with the increasing of scanning distance . The vertical error in X direction is bigger than Y、Z directions. The influence of horizontal error is parabolic trend in Y direction, and seems like the influence of roll in Z direction. The analysis shows that the four-prism tower mirror error has the significant influence on the positioning accuracy, the three links of the design and processing of the mirror, the integrated of components and the host system calibration must be strict controlled in order to eliminate or weaken this error, the study of error has the important influence on improve the airborne laser radar system positioning accuracy.