The invention belongs to the technical field of preparation of functional materials and discloses a method for high-temperature ultrahigh-pressure preparation of flaky platinum selenide. The flaky platinum selenide is synthesized at a high temperature and under a high pressure by direct adoption of elementary platinum and selenium powder as raw materials without any reaction promoters. Product purity can be adjusted by control of synthesis temperature and pressure, and block platinum selenide (PtSe2) pure in phase, excellent in crystallization performance and large in grain size is prepared. The method mainly includes steps of sample treatment, sample assembly, high-temperature high-pressure reaction and annealing for selenium removal. Simplicity of the preparation method and short preparation period are realized, and the prepared platinum selenide (PtSe2) is high in crystallinity, large in grain size and suitable for industrial large-scale production. A novel two-dimensional semiconductor preparation method is developed, reliable preparation means is provided for controllable preparation of transition metal chalcogenides and potential application of relevant two-dimensional materials to photoelectronic devices and catalysts, and a promising application prospect is achieved.