Optical constants (refractive index and extinction coefficient) of polymer semiconductor thin films are of great significance to perform optimization design. The Forouhi-Bloomer (F-B) dispersion models are applied by fitting reflectivity curves of P3HT (poly(3-hexylthiophene)):PCBM ([6,6]-phenyl-C61-butyric acid methyl ester), MEH-PPV (poly[2-methoxy-5-(2′ethyl-hexyloxy)-1,4-phenylenevinylene]):PCBM, and PEDOT (poly (3,4-ethylene dioxythiophene)):PSS (polystyrenesulfonate). The optical constants and thickness are calculated, and two reflectance curves fit well. The thickness mismatch between the fitting and measurement results is less than 3%. Based on this method, the influence of thermal annealing on the P3HT:PCBM film surface morphology and the optical constants is analyzed. The maximum refraction index of P3HT:PCBM increases from 1.95 to 2.16 in the wavelength range of 550 nm to 700 nm after the thermal annealing process at 110 ℃ and the peak of extinction coefficient shifts to longer wavelength.