Optical constants, refractive index and extinction coefficient, are significant parameters for structure design and performance optimization of polymer semiconductor thin films. We fitted the reflectivity curves of three kinds of polymer semiconductor films, such as P3HT:PCBM, MEH-PPV:PCBM and PEDOT:PSS, with the Forouhi-Bloomer (F-B) dispersion model, and got their optical constants and thickness. The fitted reflectance curves agree well with the experimental results. The thickness mismatch between the fitting and measurement results is less than 3%. Based on this method, the influence of thermal annealing on surface morphology and optical constants of the P3HT:PCBM film is analyzed. After the thermal annealing process at 110°C, the maximum refractive index of P3HT: PCBM increases from 1.95 to 2.16 in the wavelength range from 550 nm to 700 nm, and the extinction coefficient peak shifts to the longer wavelength end.