Experiments of measuring minimum miscibility pressure are conducted in order to determine the hydrocarbon gas injection pressure fast and accurately in the low permeability reservoir with lower pressure than the original reservoir saturation pressure. In combination with the actual development characteristics of Block Q in Huabei Oil Field, tubule experimental method is applied to study the natural depletion effect, minimum miscibility pressure of hydrocarbon gas flooding and gas breakthrough time under the original reservoir conditions (reservoir pressure 24 MPa and reservoir temperature 74 °C ) and the current reservoir conditions (reservoir pressure 15 MPa, lower than the saturation pressure, and the reservoir temperature 74 °C ), which can provide a basis for hydrocarbon gas injection pressure of low permeability reservoir in the block. Experimental results show that the minimum miscible pressure (62.75 MPa) of hydrocarbon gas flooding in Block Q under the current reservoir conditions is higher than that (54.77 MPa) of the original reservoir conditions. The higher the injection pressure, the later the breakthrough time of hydrocarbon gas and the higher recovery can be obtained. Miscible flooding cannot be realized under the current reservoir conditions. But the higher injection pressure can make hydrocarbon gas and crude oil to reach the near mixed-phase state so that oil recovery can be improved.