The deviation factor of natural gas under formation conditions is an indispensable basic data in the study of gas reservoir development, especially for ultra-high pressure gas reservoir. The results of high-pressure physical property experiment are accurate, yet with higher capital cost, longer cycle and heavier workload. In order to meet the needs of ultra-high pressure gas reservoir development in western Sichuan area, the measured data are obtained through field high-pressure physical property sampling and indoor experiment. This paper compares and analyzes the commonly used calculation methods of natural gas deviation factor, and selects the economic and applicable calculation methods for natural gas deviation factor. The results show that when 2.0≤Tpr≤3.0,15.0≤ppr≤30.0, the state equation of SRK model is the best, and the total error is less than 3%; in the empirical formula, ZGD method is the best, and the minimum average relative error is 1.43%. ZGD method has simple calculation process and high precision. It is suggested to be popularized and applied in this kind of gas reservoir. The calculation of natural gas deviation factor is of great significance for gas reservoir evaluation and the implementation of gas recovery process.