The invention provides a method for preparing a film of an absorbing layer of a copper zinc tin selenium (CZTS) solar cell in the technical field of solar cells. The method comprises the following steps: preparing CZTS nanoparticles; dissolving the CZTS nanoparticles in a dissolving agent to prepare CZTS nanoparticle slurry through ultrasonic dispersion treatment; coating the CZTS nanoparticle slurry on a substrate to carry out heat treatment on the substrate so as to prepare the film of the absorbing layer of the CZTS solar cell. The method overcomes the defects of expensive equipment, difficult deposition in large area and the like, and has the advantages of no pollution, simple equipment requirement and industrial large-scale production.