The invention relates to a monocrystalline gas-phase growth method for cadmium selenide with seed crystal heat conduction at top. The method solves the problems of complex equipment and easy blast ofa high-pressure bridgman method of the current grown CdSe crystal, and uncontrollable crystal direction and poor optical quality of a temperature gradient melt zone melting method and a gas-phase czochralski method. The monocrystalline gas-phase growth method comprises the following steps: 1) preparing seed crystal sheets; 2) charging to obtain a charging sealed quartz tube; 3) performing crystalgrowth: 1 activating and 2 growing; and 4) performing post-treatment, and obtaining the cadmium selenide monocrystal on the surface of the seed crystal sheet. The method has the advantage that transmittance of a middle and far-infrared wave band can reach more than 65%. The method is used for cadmium selenide monocrystalline gas-phase growth.