The invention relates to a preparation method for a CdS (Cadmium Dating Sulphide) thin film for a buffer layer of a solar battery. The preparation method is characterized in that in vacuum conditions, CdS powder is subjected to ohmic heating and is evaporated and deposited on a substrate in a sublimation manner to form the CdS thin film, and the specifics steps are as follows: selecting glass, a stainless steel metal foil or a silicon chip as the substrate, placing the substrate in a 50-100 DEG C surface active agent containing aqueous solution to be processed for 2h, then placing the substrate in acetone for carrying out ultrasound for 5-20min, placing the substrate in deionized water for carrying out ultrasound for 10-30min, and drying by using nitrogen; placing the CdS powder with the purity of 99.99% in an evaporator or crucible, fixing the evaporator or crucible in an electron beam thermal evaporation device, vacuumizing a reaction chamber to be below 10-3Pa, wherein the substrate temperature of a sample is 20-200 DEG C; controlling electric currents to be in the range of 20-30A; setting the evaporation time to be in the range of 5-15min; and carrying out annealing treatment for 1-120min at a nitrogen or helium protective atmosphere at the temperature of 100-400 DEG C, and obtaining the CdS thin film. The preparation method has the advantages that the technology is simple, the consumption is low, the thin film is more compact, smoother and more uniform, has better photoelectric property and is suitable for mass production.