Abstract: By volumetric method, isothermal absorption test of shale shows that the absorption of gas tends to decrease with pressure, even becomes negative. It is also found that the negative values for both adsorption constant, Langmuir volume and pressure occurred. Such result doesn't conform to the reality, it can make the experimental results and adsorption constant lose application value. The present paper analyzes systematically the reasons that causes "suck back" phenomenon of shale absorption experiment at the view of both shale absorption testing methods and instruments, presents the problems of free space size test and absorbed gas content test in lab, puts forward suggestions about the experimental method of absorbed gas content and apparatus for absorption testing of shale.