A novel high-precision measurement method for simultaneously measuring three-dimensional small angles is presented. The collimated beam is normally incident upon the hypotenuse-surface of rectangular-prism with splitting film. The displacements of the reflection beam and transmitted beam are measured by quadrant photoelectric detectors QD1 and QD2, respectively. The direction change of the reflection beam is double that of yaw and pitch and the direction change of the transmitted beam is double that of pitch. So the displacement introduced by roll can be measured by differential measurement based on two quadrant photoelectric detectors. This method can avoid crosstalk of straightness errors, yaw and pitch errors and enhance the anti-interference ability of the system. Feasibility of the method is verified by theoretical analysis and experiments. When the system parameters are set, the angular resolution for the yaw, pitch and roll can reach to 0.1″ and 0.3″, respectively, and the measuring accuracies are about 1″ and 2″, respectively.