To further enhance accuracy and reduce reliance on standard spectra and preset models, a novel γ spectrum quantitative method based on fuzzy C-means Gaussian mixture model (FCGMM) was proposed for low-activity environmental samples. The FCGMM algorithm was utilized to analyze the activities of ²²⁶Ra, ²³²Th and ⁴⁰K in three soil reference samples and ²³⁸U, ²³⁵U and ²³²Th in five water reference samples, respectively, as measured by the HPGe γ spectrometer (GMX40P). To compare against the Gaussian fitting method (1.73%~14.99%, 8.59% on average) and the first derivative method (2.67%~16.29%, 9.81% on average) by calculating the relative errors, the FCGMM algorithm reduced relative errors by 1.87%~144.15% and 9.81%~173.61%, as well as average relative errors by 50.17% and 56.37%, respectively. For activities below 40 Bq, the FCGMM algorithm demonstrated significantly higher accuracy. The results indicate that the presented method can serve as a reference in low-activity γ spectrum analysis, providing rapid and accurate performance without the need of preset models.