Abstract: On the basis of the asymmetric double sigmoid function, a simple method was developed to fit the LSC (liquid scintillation counting) spectra of mixed ⁹⁰Sr/⁹⁰Y samples under different quench levels using the software Origin. Because the external standard quench index is usually affected by the statistical fluctuations significantly for a sample with high radioactivity, a quench index is defined, which is based on the characteristics of ⁹⁰Y spectrum and will not change with the sample radioactivity. The defined quench index has been correlated with the parameters which reflect the position and shape of the ⁹⁰Y spectra. Based on the correlation, the LSC spectra for mixed ⁹⁰Sr/⁹⁰Y samples under different quench levels have been fitted, showing good agreement with the results from experiment. This method is promising to be applied to simultaneously determine the radioactivities of several radionuclides under different quench levels by LSC.