Gaseous Z-factor is key data that is used to calculate the reserves of gas reservoir, we can generally make certain Z-factor by Z-factor charts, experience methods and experimentation. However,because deep layer and ultrahigh pressure gas reservoir has high temperature and high pressure, we are devoid of proof of practical measurement whether these conventional methods are applicable. Experiment of high temperature and high pressure show Z-factor and critical pressure are greatly excessive lowpressured gas reservoirs under ultrahigh pressure condition, which has gone beyond applicable scope of Z- factor charts, so conventional methods are usually inapplicable; Z-factor and pressure has good linearity relation under high temperature and high pressure condition, therefore, it is feasible for Z-factor of ultrahigh pressure gas reservoir to apply experiment result and linearity relation to extrapolate the original stratum pressure condition, which will greatly reduce the request of experiment equipment, and predigest the process of experiment.