A laser autocollimator based on transmission grating and combined reflector is proposed to simultaneously measure a three-dimensional angle. The optical configuration of the proposed autocollimator is designed, and a mathematical model for measuring a three-dimensional angle is established. The three-dimensional angle is obtained by detecting the change in the direction of the three measurement beams generated by grating diffraction and reflected by a combined reflector. The experimental setup based on the proposed autocollimator was constructed, and a series of experiments were performed to verify the feasibility of the proposed autocollimator for precision angle measurement. The experimental results showed that the measurement resolution of three-dimensional angles is better than 0.01", with measurement repeatability of yaw, pitch, and roll angles being 0.013", 0.012", and 0.009", respectively.