The phase modulator is a key component of the beam synthesis unit in the coherent synthetic aperture telescope, and the accurate measurement of the motion information of the phase modulator is one of the key technologies for phase modulator control. The three-degree-of-freedom (pitch, azimuth, and piston displacement) measurement method based on the plane normal vector is proposed. The angle and displacement information are solved according to the change of the normal vector. The proposed method is described in detail and all the factors influencing measurement accuracy are analyzed. The experimental results show that the displacement range is 0-3 μm when the rotation ranges of the rotation angles α and γ are 0-0.44 mrad and 0-0.28 mrad, respectively. Within the measurement range, the angle error is less than 3.3 μrad and the displacement error is less than 50 nm. The proposed method has the advantages of simple structure, high measurement accuracy, and so on, which can be widely applied in the three-degree-of-freedom measurement of phase modulators and others.