Abstract: Generally, pseudo pressure for real gas is calculated by simplifying pseudo pressure as pressure square or using numeri- cal integration directly, which can not exactly represent the pseudo pressure for real gas. With a gas composition of the gas ield as an example, the paper calculates viscosity μ and compressibility factor Z, and presents a new empirical formula of the product of μ and Z with pressurep and temperature t going on, and obtains pseudo pressure for different dry gas with deinite pressure and temperature. The results are received that Zμ increased with the respect to pressure, i.e. Zμ has a parabolic increase in an area of low pressure and a linear increase in an area of high pressure, and that gas composition has little effect on pseudo pressure calculation.