We used the traditional hydro-thermal synthesis method to generate the first photochromic pyrimidinium/polyoxometalate (POM) salt material (C4N2H4–C4N2H3) Mn(H2O)60.5{SiW12O40}0.5ˑ5H2O in situ by using silicotungstic acid and pyrimidine as raw materials. The solid UV diffuse reflectance spectrum of compound 1 was shifted before and after illumination. There was a new broad absorption peak appeared near 370 nm and 624 nm after illumination. With the increase of irradiation time, the peak at 624 nm will shift to 588  nm at which the sample reached light saturation. The color of the sample changed from yellow to grayish green. By studying a series of test: EPR, XPS, UV–Vis, X-ray single crystal diffraction, and density of states (PDOS) calculation, we conclude that the charge transfer mechanism of photochromism may be due to: the generation of pyrimidinium free radicals by light-induced, photochemical reduction of polyoxometalates and the interaction of two aspects. This work enriches the family of photochromism and its theory. Experiments for the first photochromic pyrimidinium/POM salt material (C4N2H4–C4N2H3)Mn(H2O)60.5{SiW12O40}0.5.5H2O and in-depth explored for the mechanism of photochromism. Display omitted •The first photochromic pyrimidinium/polyoxometalate salt material.•Photochromic mechanism under the action of bifunctional groupsphotochromism.•Reversible photochromic materials at 370 nm, or between 657 nm and 588 nm.