The invention discloses a method for assisting in deposition of a perovskite crystal film through steam annealing and belongs to the photoelectric field. According to a film annealing mechanism, control on a growth process of the perovskite crystal film is realized through creatively changing temperature difference between solvent steam and the perovskite crystal film, moreover, control on the perovskite crystal film morphology is realized through changing application amount of a solvent or steam pressure of the solvent, and auxiliary steam annealing can be single-time annealing or can be multi-times continuous or cycle annealing. The method is advantaged in that compared with a routine annealing method for devices, efficiency of a flat heterojunction solar energy cell device of the perovskite film based on solvent steam annealing processing is improved to a great degree, efficiency reaches 15% under the non-modified cathode condition, and efficiency is higher than similar device structures reported in most of documents.