The invention discloses an automatic peak searching method for seawater radioactivity detection. The method comprises: five-point smoothing of data of all channels is carried out; second-order derivatives for all data are calculated, all second-order derivatives in a preset radionuclide existence zone range are traversed to be negative data points as preset peak positions; left and right boundaries of the preset peak positions are calculated; Gaussian function fitting of original data in the left and right boundary ranges is carried out, the full width at half maximum of each obtained fittingfunction is compared with a preset full width at half maximum of the radionuclide, data exceeding the range of plus or minus 5% are directly eliminated, and the preset peak position with the fitting function having the full width at half maximum closest to the preset full width at half maximum of the radionuclide is determined as a to-be-searched radionuclide peak. The disclosed automatic peak searching method is free from the accumulation time restriction as well as the restriction of the interference of the marine environment; overlapped peaks can be identified; and some obvious false peakscan be filtered automatically. Therefore, the peak searching accuracy is improved.