The invention provides silver selenide/selenium silver sulfide core-shell quantum dots, an electrooptical modulator and a preparation method. According to the preparation method, octadecene-selenium is used as a selenium precursor, silver selenide quantum dots are prepared; the surfaces of the silver selenide quantum dots are coated with selenium silver sulfide alloy shells to passivate the surfaces of the silver selenide quantum dots, reduce the surface defects of the silver selenide quantum dots, construct a gentle interface barrier, inhibit auger recombination of the silver selenide quantumdots, and eliminate a local electric field in a random direction; the toxicity of the silver selenide/selenium silver sulfide core-shell quantum dots can be ignored, the silver selenide/selenium silver sulfide core-shell quantum dots are safe and reliable; perhydropolysilazane is adopted to be converted into a silicon dioxide packaging layer of the silver selenide/selenium silver sulfide core-shell quantum dots in the atmosphere and at the room temperature, and the electrooptical modulator which is continuously adjustable in working wavelength along with the size of the quantum dots in a near-infrared second window, large in modulation depth, stable in performance, long in service life, simple in structure and environmentally friendly can be prepared.