In some electronic devices associated with power grid three-phase voltage,it is very important to obtain the real-time fundamental wave phase of the power grid voltege accurately. Aiming at the condition of the power grid appearing unbalanced three-phase voltage, this paper put forward a design scheme of three-phase digital phase lock loop based on single second-order generalized integrator and do the simulation by Matlab/Simulink. The simulation results show that the phase lock loop can track the phase of three-phase power grid accurately when power grid appear three-phase drop and unbalanced three-phase voltage. The design principle is clear, simple structure and easy to implement. It is easy to transplant in FPGA and constitute the entire hardware integrated PLL; it also can be transferred in DSP and constitute a software phase locked loop.