The gas-insulated switchgear is always affected by the white noise during partial discharge detection. A stationary wavelet transform noise filtering method based on a new noise threshold rule is proposed to filter out the white noise in partial discharge. The lower control limit and upper control limit of the wavelet coefficient are determined by statistical process control theory, and they will be updated iteratively according to the statistical characteristics of wavelet coefficients in the method. Then the noise threshold level of the signal is obtained through the upper and lower limits, and the white noise in the signal will be reduced adaptively. Since the down sampling of traditional wavelet transform will not appear in the stationary wavelet transform, the feature of the partial discharge signal will be more complete. In this paper, the noise suppression of three 5dB noise-stained partial discharge signals is carried out. The signal-to-noise ratio reaches 19.1433 dB, and the root mean squared error is maintained within 0.03 after noise reduction. In addition, the noise rejection ratio is 17.1769 for signals from a laboratory. The noise in the partial discharge can be better suppressed by the proposed algorithm. In addition, the feature of the waveform is obvious and the distortion is low.