Abstract: Huagang Formation in Gas Field A of Xihu Sag is deeply buried and has undergone complex diagenetic processes, leading to severe reservoir densification. Relatively high-quality reservoirs are locally developed with strong heterogeneity, which substantially impacts gas field exploration and development. This study focuses on the main controlling factors of high-quality reservoir development in Huagang Formation of Gas Field A from two aspects: sedimentation and diagenesis.The results indicate that sedimentary facies provide the foundation for high-quality reservoirs development. Huagang Formation belongs to the braided river delta facies, characterized by thick and continuous sand bodies, laying the material basis for the development of high-quality reservoirs. Sandstone reservoirs with relatively coarse grains in the main distributary channels and the middle-lower parts of channels exhibit the best physical properties. Diagenesis plays a key role in the development of high-quality reservoirs, among which dissolution and chlorite coating are critical factors controlling the formation of high-quality reservoirs in Huagang Formation. The research results provide an important reference for further deepening the understanding of the reservoir formation in Gas Field A and optimizing the deployment of subsequent well locations.