The invention belongs to the technical field of the preparation of transmission electron microscope samples of soft brittle phototransistors, and relates to a method for preparing a transmission electron microscope sample of a soft brittle phototransistor, in particular to a method for preparing a transmission electron microscope sample of a soft brittle photo semiconducting transistor. The method is characterized in that a hard brittle semiconductor with micro Vickers hardness of over 8 GPa is used as a protective layer of the soft brittle phototransistor, wherein the roughness Ra of the ultra-smooth surface of the protective layer is below 1 nanometer, and the thickness is below 650 micrometers; sand paper of 120 to 800 meshes is adopted in the coarse grinding stage, and a membrane is ground by adopting diamond of between 0.1 and 30 micrometers in the accurate grinding stage; the protective layer is bonded with the sample by adopting a hot setting adhesive which is insoluble in an organic solvent, and a first surface of the ground transmission electron microscope sample is bonded with a substrate by adopting a hot setting adhesive which is soluble in the organic solvent; and in the final stage of accurate grinding, the transmission sample and a thin area of the protective layer are obtained by a stress-free grinding method. The method has the advantages that the preparation of the transmission electron microscope sample of the soft brittle phototransistor is realized, and high-resolution transmission electron microscope atomic images are obtained.