To cope with the problem of the voltage-zero drift and the phase delay in zero crossing phase lock, a precise phase lock technique used for grid voltage zero-crossing is proposed. This method removes the zero drift in the ADC sampling result by using PQ filtering; and then sets a zero phase locking schedule artificially, in order to foresee the zero-crossing point of the grid voltage; lastly, through software self-adaptive compensation algorithm, the precise detection of grid voltage zero point can be realized. The design principle and calculation process of this method are introduced. This method is proved to be correct by experiment on the platform of the self-charging voltage sag compensator without rectifier.