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.