The invention discloses a copper indium gallium selenide thin-film solar cell uniform in gallium distribution and a preparation method thereof. The plasma enhancement selenylation etching composite technology is introduced in the selenylation reaction process, the activity of selenium steam is improved by means of plasma, meanwhile an alloy prefabricated layer is etched by means of the plasma, redundant selenium in the alloy prefabricated layer is removed, the surface area of the alloy prefabricated layer is increased so as to achieve better diffusion and permeation of activated selenium steam, reaction of the activated selenium steam, copper, indium and gallium is made more sufficient, and uniform distribution of gallium in a thin film is achieved. All elements in components, especially gallium, of the prepared copper indium gallium selenide thin film are uniform in distribution, and conversion efficiency of the prepared copper indium gallium selenide thin-film solar cell is greatly improved.