Problem of voltage sag is one of the serious power quality problems often encountered in the power system, the traditional strong tracking filtering algorithm on the voltage sag detection of judgment on the filtering divergence threshold setting is small, there will be a larger probability to produce fading factor and cause of filter gain adjustment, eventually leading to the system state estimation is not smooth. In order to quickly and accurately detect voltage sag characteristics, including transient drop value, start and stop time and phase jump. In this paper, a voltage sag detection method based on improved strong tracking filter algorithm is proposed. On the basis of considering the effects of harmonics, DC offset, non-integral harmonics and noise, an improved strong tracking filter model including DC component, fundamental wave and harmonic component is established. The starting and ending time of voltage sag is indicated by the abrupt change of reduction factor, and the transient drop value and phase jump are detected by the improved strong tracking filtering algorithm. The simulation results show that the improved strong tracking filtering algorithm is more robust than the traditional strong tracking filtering algorithm, and can effectively maintain the filtering accuracy and the smoothness of state estimation, and the convergence speed is faster.