Frequency stability is an important indicator to evaluate the performance of a coherent population trapping (CPT) atomic clock, and the type and pressure ratios of the buffer gas compositions inside an atomic vapor cell are main factors influencing frequency stability. In this study, the temperature frequency shift caused by buffer gas compositions, i.e., Ar and N2, was simulated and analyzed theoretically, and the temperature shifts caused by different pressure ratios of buffer gas compositions were tested experimentally. The optimal ratio of buffer gas compositions in the atomic vapor cell was determined based on the theoretical and experimental results, and the corresponding operation point for the minimum temperature frequency shift was found. The research results provide a valuable reference for selecting the pressure ratio of buffer gas compositions and the operation temperature of the atomic vapor cell for a CPT atomic clock.