Using a pair of orthogonally linearly polarized beams as the light source, the Soleil-Babinet compensator to offer a continuous phase retardation and a lock-in amplifier as the data collector, a collinear heterodyne interferometer system was presented, which is consisted of a narrow band semiconductor laser with fiber-coupled output and two acousto-optic modulators. The phase characteristics of this system were researched and the influence of the laser beams drift, amplitude modulation and laser line width on the phase measurement was analyzed. The results show that the phase resolution is 0.3611 μm, the phase sensitivity is 27.386°/mm, and the measurement error of this system is 0.090 rad. And this system we proposed can eliminate the affect of the amplitude modulation of acousto-optic modulator and the error caused by laser beam drift. Because the two beams are perfectly collinear, the noise resulting from the environment is also reduced.