In HVDC transmission project, the high-precision phase-locked loop is used to trace the real-time grid phase, when the simulated phase is equal to the trigger angle, the trigger pulses are generated and the converter valves are triggered, so the instantaneous frequency tracing should be fast and accurate, but the traditional frequency measurement algorithms could not meet both the speed and the accuracy requirements simultaneously. This paper designs a new grid instantaneous frequency tracking algorithm. This algorithm transforms the three-phase voltages of grid by Clarke transformation; after filtering the harmonics and negative sequence components by using the FIR, it calculates the grid instantaneous frequency, and tracks grid synchrounous frequency with phase-difference method. According to the simulations and experiments, the algorithm can precisely and quickly trace the grid instantaneous frequency, and the accuracy is up to ±0.005 Hz.