Abstract: The dispersion of temperature and pressure is very widespread in the process of exploiting HTHP gas reservoir. At present the calculation of gas compressibility factor for wide range of temperature and pressure combination is very immature. the most of different calculating models of gas compressibility factor,which are usually used in the petroleum industry,were discussed in this paper. Systematically comparing and estimating has been done to all the models, including Dranchuk-Abu-Kassem model, Hankinson-Thomas-Phillips model, Hall-Yarbough model, Dranchuk-Purvis-Robinson model, Li Xiangfang model and Sarem model. The author presents a view that different calculating model for gas compressibility factor has its the most applicable range of temperature and pressure combination in which the error of calculation is smallest. Then the paper recommends a proper calculating routine for gas compressibility factor in a wide range of temperature and pressure combination. The plates of gas compressibility factor have verified all conclusions in this paper.