Abstract: A relatively simple and accurate photometric method was presented to calculate the optical constants of the weak absorption coatings on weak absorption substrate materials . And the optical constants of the JGS1 fused silica substrate, MgF2 and LaF3 films were calculated according to the above method. The results show that, at the wavelength of 200 nm, the absorption of the JGS1 fused silica substrate is relatively obvious with the extinction coefficient of 10-8 level. Therefore, the weak absorption of the substrate should be taken into account to improve the calculation precision of the film’s optical constants.