The invention discloses a phase-changing transformation method of a coal reservoir, and belongs to the technical field of the transformation of coal reservoirs. According to the phase-changing transformation method of the coal reservoir, through injecting sealing liquid, phase changing liquid, the sealing liquid and cooling medium liquid in sequence into an eyelet in the coal reservoir, the cooling medium liquid is enabled to conduct cold energy to the phase changing liquid, the phase changing liquid is enabled to generate phase change, and swells in volume, thus, a reticular crack is formed in the coal reservoir around the eyelet at a low temperature and under swelling stress generated by the phase changing liquid, branches of the reticular crack are more, exploiting channels for coal-bed gas are increased, and the exploitation of the coal-bed gas adsorbed in the coal reservoir is more facilitated; moreover, by using the method, the coal reservoir is enabled to expand from the eyelet to generate the crack, does not bear the scouring of an external object, therefore, coal powder generated by a coal rock is reduced, thus, the blockage of the crack is avoided, the flow conductivity of the crack formed by the phase-changing transformation method of the coal reservoir is enabled to be stronger, the discharge and exploiting work of the coal reservoir after phase-changing transformation is facilitated, and the exploitation efficiency of the coal-bed gas is improved.