The present invention discloses a method for correcting a physical ultra-violet light intensity measuring instrument, comprising the following steps: forming I3<-> through the optical function of the ultraviolet to the mixed solution of iodate and iodide, and measuring the concentration of the I3<-> ; and through a parallel light tester and at least three different 254nm-single wavelength UV light intensity experiments, obtaining a calibration curve or calibration equation method through regression analysis so as to the physical light intensity measuring instrument. The iodide-iodate photochemical UV intensity measuring method which aperiodically corrects the physical ultra-violet light intensity measuring instrument is reliable, economical and simple; besides, the results are accurate, with high repeatability; therefore, the method is available to the relevant research units, UV lights manufactures and quality inspection departments to measure the 254nm-UV intensity.