As a critical technique for increasing the brightness of narrow-linewidth fiber lasers, coherent polarization beam combining can achieve common aperture joining of multiple lasers, while maintaining higher beam quality and linear polarization state. A polarization control system of combined lasers was investigated based on the linear phase locking technique. A physical model for optical homodyne polarization detection and a mathematical equation for linear phase-locking loop were established and analyzed in detail. The polarization phase of combined lasers was checked by employing high-precision optical homodyne method and then fed back to phase lock lasers in real time. A linear polarization laser was output with the power of 279 mW. After phase locking, the polarization extinction ratio of combined lasers attained 19.3 dB and the control bandwidth was 39.6 kHz. The residual phase noise is 7×10-4 rad/S(1 Hz) and 3×10-4 rad/S. When the power was increased to 1 W, the polarization extinction ratio was -15 dB, which was restricted by power-induced phase noise and spatial mismatch of beam spots.