Power quality disturbance signal detection method based on discrete orthonormal S-tranform is proposed to detect the begin-end moment of disturbance signal accurately. Based on the traditional S-transform, this paper discretizes the signal combined with the fast Fourier transform, then introduces band center, frequency bandwidth and time variables to improve the algorithm, constructs the basis function vectors to obtain the discrete orthonormal S-tranform (DOST) coefficient matrix, finally detects the disturbance moment of the disturbance signal by finding the catastrophe point of the discrete orthonormal S-tranform coefficient matrix. The analysis results of this method are compared with that of traditional S-transform through simulation experiment. Results show that the DOST method can detect the begin-end moment of the disturbance signal precisely and effectively.