The invention discloses a photochemical reaction device and a method for synthesizing vitamin D3 by using narrowed high-voltage mercury lamp wavelength and one- step optical isomerism, relating to a vitamin D3. It comprises container, internal pipe, mercury vapour lamp, mediolateral jacket and collecting bottle, the mercury vapour lamp is installed in internal pipe, external jacket is set inside of internal pipe, internal jacket is installed outside of internal pipe, internal jacket is installed between internal tubular lamp and external jacket, the irradiation liquid export of container is connected with collecting bottle. Said method comprises following steps: preparing 7- desmesterol solution and dissloving it in ethanol; evacuating and ventilating nitrogen gas, preparing solution with sodium tungstate or acetate of lead, feeding it into external jacket as filtering liquid, preparing solution with cobalt sulfate, nickel sulfate, sulphuric acid and water, feeding it into internal jacket as filtering liquid, feeding 7- desmesterol solution from feed inlet through condenser circulation for photochemical reaction, ventilating nitrogen gas from feed inlet to mix with 7- desmesterol solution, discharging solution after photochemical process into collecting bottle.