Abstract: In order to estimate the number of coherent signals more effectively，proposes a Gerschgorin radii estimator that is designed for small array.This method has been proved in theory and applied successfully in the signal processing of shallow water Bathymetric Side-scan Sonar System（BSSS） whose array is small.In the Qiandao lake test of BSSS，this Gerschgorin radii estimator could detect the false signal of tail flow of boats effectively with less computer burden than that based on spatial smoothing and without cutting off the array aperture.