In order to investigate the physical meaning of critical desorption pressure in undersaturated CBM（coalbed methane） reservoirs

the traditional critical desorption pressure calculation method and the liquid phase adsorption theory commonly used in field are introduced based on the status of the reserves of CBM

and the origin of the “undersaturated” phenomenon under the premise of gas phase wet is discussed. On the basis of the modified gas phase adsorption

the relationship between the critical desorption pressure and the capillary pressure in matrix pores is explained

and a new calculation method of critical desorption pressure is obtained. By comparing the advantages and disadvantages of the four methods in error analysis

theoretical completeness and operability

it is considered that the new calculation method proposed is relatively complete in theory

has strong operability and is easy to calculate

and obtains a reasonable result in an example. It can be concluded that CBM adsorption equilibrium and critical desorption phenomenon can be interpreted as the result of the interaction between reservoir pressure and capillary pressure under the premise of coalbed gas phase wetting. The improved gas phase adsorption theory and the new calculation method of critical desorption pressure are worth the further study.