Abstract: Following both the conventional and unconventional oil and gas accumulation theories, and upon the basis of the experimental and testing results of the reservoirs and fluids, this paper studied the characteristics of gas reservoirs and the controlling factors on hydrocarbon accumulation in the Huagang Formation in the Y structure of Xihu Sag. And the following research results are obtained. ① there is a vertical distribution pattern of gas accumulation from the middle and lower tight sand gas to upper conventional reservoir gas in the Huagang Formation of the study area. ② Influenced by the sealing of matched fault and cap rock system to oil-gas migration through faults and by lateral migration, the conventional gas reservoirs with a normal gas-water contact are formed and distributed at the top of fault-anticline traps below the regional caprock. ③ The tight sand reservoirs are characterized by separated source-reservoir system, low gas saturation, complex gas-water relationship, and antecedent tight sand reservoir. Reservoirs which have different porosity and located in different structural heights have different gas saturation. The result has an important significance for the study of the accumulation models of the conventional and unconventional gas reservoirs in the Xihu Sag and will provide a guidance for exploration of the tight sand gas.