Abstract: The Huagang Formation in Ningbo tectonic belt of Xihu Sag is a set of tight sandstone gas reservoir. The study on the compactness of the reservoir and its main controlling factors is signifcant to further exploration and development. Based on some of the previous researches, thin section analysis and geochemical parameters of core samples, the author studied the petrologic characteristics, physical property, diagenesis and controlling factors for the tight sandstone reservoir of Huagang Formation. It's believed that the Huagang Formation sandstone is a typical low porosity and low permeability and ultra-low porosity and permeability reservoir. The rock type is feldspathic lithic quartz sandstone, and the secondary porosity is the main reservoir space. The main factors controlling the compactness of the reservoir are compaction and cementation. The mathematical geology model shows that more than 80% of the pore loss are related to compaction, and the average pore reduction rate of cementation is 7%.