Volumetric method and gravimetric method are two important methods for shale gas adsorption experiment. Compared with the gravimetric method based on magnetic suspension balance（MSB），the instrument of volumetric method has simple structure and principle，and low cost. The volumetric method is the main means to evaluate the shale gas adsorption experiments in China. However，the biggest problem of the volumetric method is that the calculation of the gas compressibility factor is inaccurate，especially when this method is used to calculate the adsorption amount at high pressure. In order to solve this problem，the compressibility factors of seven complex gas state equations were calculated through programming，and the calculated results were compared with those calculated in the Chemistry part of NIST database in USA. The results show that the gas compressibility factor calculated by the Setzmann equation is accurate at both high and low pressures， which solves the shortage of the volumetric method at high pressure. The actual shale gas isotherm data was interpreted by the volumetric method and the improved volumetric method respectively. The result shows that when the equilibrium pressure exceeds 5 MPa，the adsorption amount difference increases with the increase of equilibrium pressure，the maximum adsorption amount difference is 0.426 5 mL/g at equilibrium pressure of about 30 MPa，which indicates the importance of accurate calculating the gas compressibility factor. The adsorption isotherm of the shale gas was fitted by the adsorption theory model. The results show that Toth adsorption model has the highest accuracy.