Beams combination is an important way for development of high power solid state lasers,and the phase control electronics contributes greatly to the success of the coherent combining. The method of phase detection for beamlets in a 4 rectangle spot beams array is analyzed,and its mathematical model of coherent combination under phase-locked condition is established. A high-speed wavefront sensor is used to sample the wavefront of one beamlet in laser array,and phase difference between the beamlet and reference beam is detected by the way of heterodyne method from the beat frequency signal,which results from the interference of the reference beam with each beamlet. The phase difference is used to actively control the phase modulator of the beamlet,and then all of the output phases are exactly aligned. Some simulated waveform signals changing versus time are presented during the locking loop is closed,and it is also simulated that the intensity distribution of combined far-field spots changes with phase difference and random aberration among these beamlets,which is compared with that when the locking loop is opened. It is concluded from the simulated results that all output phases of each beamlets can be exactly aligned when the locking loop is closed. The control precision and condition of closing-loop are also analyzed.