Abstract: In order to predict the performance of the detection of periodical modulation of ship noise in Gaussian noise background, a simple formula (19) for calculating the MDS (Minimum Detectable Signal) for the detection of periodical modulation of periodically locally stationary narrow-band signal in stationary Gaussian white noise is derived. The formula shows that the detection performance depends on the following four factors:depth of the periodical modulation of the signal; bandwidth of a pre-processor; bandwidth and average time of a post-processor. If background noise is not stationary, the detection performance will be worse than that predicted by formula (19). With the view of reducing the error, formula (19) is modified by using the parameter B_eqN which is measured in experiments to replace B in formula (19). In the case of nonsltationary background noise, the detection performance depends not only on the above mentioned factors, but also on the nonstationariness of background noise.