Abstract: After the wavelet transform to γ-ray spectra, modulus maximum of statistical fluctuation and characteristic peak vary with the transforming scale. According to this characteristic, an arithmetic to detect low signal in γ-ray spectra was designed. Characteristic peak of ¹³⁷Cs in soil sample is the object in this work. The result proves that this arithmetic can detect low signal, and the intensity of signal can also be calculated by fitting intensity with modulus maximum.