As a secondary alteration process in oil reservoirs and a critical mechanism for light oil and condensate formation, gas washing is predominantly governed by temperature and pressure. However, the dynamic effects of these two factors on the light hydrocarbon composition of residual oil remain unclear. This study developed a methane-pressurized gas washing simulation apparatus that can maintain predetermined pressure and temperature conditions throughout the experiment. The aim of this study is to investigate the variation patterns of light hydrocarbons under different temperature and pressure conditions, and to further evaluate their potential influences in identifying the type and depositional environments of source organic matter, quantifying thermal maturity, and identifying secondary alteration processes. By establishing the relationship between the remaining mass fraction of light hydrocarbons and time, the loss rate constants for individual components were obtained. Comparative analysis revealed that the loss rates of light hydrocarbons increased significantly with rising temperature during gas washing, but were suppressed with increasing pressure. Among the parameters indicative of source rock type and depositional environment, the Mango parameter K1 and the C7 ternary diagram were less affected by gas washing and changes in temperature and pressure. In contrast, parameter K2 and Halpern's oil-gas correlation parameters (C7-OCSD star diagram) underwent considerable alterations. Gas washing resulted in an increase in both MCH/(MCH + nC7) and Tol/(Tol + MCH) ratios, a trend opposite to that induced by increasing maturity. Furthermore, elevated temperature significantly promoted an increase in the MCH/(MCH + nC7) ratio, whereas increased pressure exerted an inhibitory effect on this ratio. The variation in secondary alteration parameters (Tol/nC7 and nC7/MCH) is consistent with classical theory: gas washing leads to an increase in Tol/nC7 and a decrease in nC7/MCH. Furthermore, higher temperatures result in a smaller increase in Tol/nC7, while greater pressure leads to a more pronounced increase. These findings enhance the understanding of how temperature and pressure govern gas washing mechanisms, providing critical insights for more accurately identifying hydrocarbon origin, maturity, and secondary alteration processes in complex geological contexts.