Abstract: Rock physics models serve as a bridge connecting reservoir properties and elastic parameters. However, conventional rock physics modeling is usually carried out under normal temperature and pressure conditions, ignoring the influence of in-situ temperature and pressure on petrophysical properties. Understanding the relationships between petrophysical parameters, e.g., porosity and water saturation, and seismic parameters under in-situ reservoir conditions (high temperature and high pressure) is of great significance for the accurate identification and quantitative interpretation of sweet spots. The case study deals with the tight sandstone reservoirs of the Huagang Formation in Block X of the Xihu sag, and uses 55 core samples from 5 wells for dry rock velocity and porosity measurements under ambient and high-temperature and high-pressure conditions, followed by the analysis of velocity variations with changing saturation. Under the constraint of high-temperature and high-pressure rock physics experiments, this study calibrates the relationships among the elastic parameters of the rock matrix, dry rock skeleton, and saturated rock under in-situ conditions, and establishes a rock physics modeling workflow based on the KT and WPH models. Furthermore, it identifies the rock physics response characteristics of sweet spots in the tight sandstone reservoirs of the Huagang Formation, constructs rock physics templates, and determines the diagnostic elastic parameters for sweet spot identification. The results show that the rock physics model based on the KT and WPH models achieves high reconstruction accuracy for the tight sandstone reservoirs of the Huagang Formation, with the reconstructed P-wave and S-wave velocity errors less than 5%. The diagnostic sweet-spot parameter is the P-wave to S-wave velocity ratio, and a ratio below 1.65 can effectively indicate sweet spots.