Based on the microseismic monitoring data at the left bank abutment slope of Jinping first stage hydropower station, the microseismic activity characteristic is analyzed. The S transform(ST) is used to decompose waveform, and this method is also applied to process the waveforms in the impoundment and construction periods of the left bank abutment slope to investigate the time-frequency characteristics of microfractures. The results show that: (1) Compared to the construction period, the rock mass of the abutment slope tends to be stable in the impoundment period, and the microseismicity is intimately related with the impoundment and geological structures. (2) The ST concentrate the advantages of short-time Fourier transform(STFT) and wavelet transform(WT), it has desirable characteristics in the time-frequency analysis of non-stationary signal. (3) The time-frequency characteristics are more stable than waveform characteristics; and the waveform decomposition based on ST can successfully reveal the damage information of seismic source location in the waveform. (4) Compared to the construction period, the frequency and mono-vibration proportion of microcracking signals of the left bank abutment slope in the impoundment period increases; and the duration decreases. These phenomena are due to the enhancement of rock mass integrity and strength at the left bank abutment slope during the impoundment period. Thus the microseismic signal shows different time-frequency characteristics. The time-frequency parameters of microseismic event can reflect the microfracturing information of rock mass and provide references for the stability evaluation.