The shale gas and coalbed methane are very important unconventional natural gas resources in China. The isothermal adsorption parameters are used in resource evaluation. The determination of the parameters needs to be solved by setting up isothermal adsorption curve through the adsorption experiments. According to the characteristics of the adsorption experimental device，the current adsorption instruments can be divided into weighing instrument and pressure testing instrument. When a rock sample is loaded into a sample cell or test barrel，the void volume needs to be calibrated before calculating the molar amount of free methane in them，either by pressure testing or by weighing method. The sum of the molality of methane in the void volume and the molality of methane absorbed from the rock sample is equal to the total molality of methane injected at a certain stable pressure. Therefore，it is very important to accurately calibrate the void volume size.As helium（He）is stable in physical properties and does not adsorb with rock samples，helium gas is usually used as the medium to calibrate the volume of free space. Based on free state equations and instrumental principles，this paper provides the weighing method and pressure testing method to calibrate the void volume by using He. Application of an example indicates that the method presented in the paper is effective.