Abstract: The capacity expansion of underground gas storage in water flooding sandstone gas reservoir form can be realized through water flooding. But influenced by the reservoir heterogeneity and water invasion, the storage capacity and the working gas volume is mostly less than the designed value during the actual operation process. In order to optimize the injection production effect of gas storage, multi-rounds long core water-gas mutual flooding experiments were carried out to analyze the pressure field distribution and capacity utilization characteristics during the process, and to study the factors that affect the expansion. The study shows that with the more rounds of injection, gas displacement becomes more difficult, and the displacement result gradually becomes poorer, tending to the maximum storage capacity. The reservoirs located in far well area cannot effectively participate in storage operation. The storage equilibrium pressure is less than the upper limit pressure after the injection; and it is more than the lower limit pressure after gas recovery, which causes the actual storage capacity and the working gas volume is always less than the designed value. Restricted by the reservoir pore throat heterogeneity and gas drive pressure gradient, and influenced by side water cycle, the capacity expansion increased by the displacement is limited. After injection-production simulation, the capacity utilization rate is only 30%, and the working gas volume ratio is also only 21.6%.