As an important indicator of power quality, how to accurately and quickly identify the voltage sag characteristic quantity information is the key issue for the situation of large-scale access to voltage sag sensitive load in power systems. This paper starts with the concept and characteristics of voltage sag and explains the different voltage sag detection methods in the time domain and transform domain. Methods based on time domain such as the RMS detection method, residual voltage detection method, two-point detection method and d-q coordinate component detection method are analyzed in detail considering their timeliness, stability and practicability. Wavelet transform, short-time Fourier transform and other methods in transform domain are also included. Finally, typical strategies in both time domain and transform domain are selected for simulation analysis. The simulation results are consistent with the theoretical results. Finally, this paper gives a brief summary of the main detection methods listed in the paper.