The invention relates to a preparation method of an adjustable luminescent CsPb2Br5 nanosheet, a product and application thereof. The preparation method comprises the following steps: under nitrogen protection, mixing cesium stearate, octadecene and oleic acid to prepare a caesium oleate solution; additionally, mixing lead bromide, octadecene, octylame and oleic acid, heating and stirring to obtain a PbBr2 precursor solution; and then adding the caesium oleate solution into the PbBr2 precursor solution, mixing and stirring to obtain the CsPb2Br5 nanosheet. Different sizes of CsPb2Br5 nanosheets are obtained by regulating and controlling the reaction temperature of preparing the PbBr2 precursor solution to be 100 to 140 DEG C, and the photoluminescence gradually changes into green light from blue light. The obtained CsPb2Br5 nanosheets can be used for preparing light-emitting diodes, solar cells or photoelectric detectors, and the light-emitting efficiency of high-monochromaticity green light LEDs constructed by preparing the CsPb2Br5 nanosheets reaches 34.49 lm/W.