The single-zone heat-release rate algorithm is widely used in combustion heat-release diagnosis in gasoline engine. However, it is difficult to reasonably estimate the specific heat ratio and heat transfer, which affects the reliability of the analysis results. Based on the single-zone heat-release rate model, a specific-heat-ratio estimation method considering the influence of heat transfer is proposed, which calculates the polytropic exponent during compression and expansion, and interpolates it based on the gas temperature in the combustion interval to obtain the variable specific heat ratio for the calculation of the total heat release. From the perspectives of the energy released during the cycle and the combustion control parameters, the proposed method is compared with the existing method by using the experimental data of the gasoline engine. The results show that the total cumulative heat release calculated by this method is more accurate, the combustion end point is advanced, and the correlation between the combustion control parameters is significantly improved, indicating that this algorithm can describe the combustion history in the cylinder more accurately.