Considering the quantum confinement effect in silicon nanoparticles, the effect of the mean silicon nanoparticle size on optical band gap, optical transition oscillator strength, and the temperature dependence of optical absorption, a model is introduced to analyze the optical absorption of silicon-nanostructure thin film with a certain size distribution of silicon nanoparticles. The results show that the band gap increases and the absorption spectra curves are shifted vertically with increasing temperature, which is like in bulk silicon. The comparative analysis of the simulation results and experimental data of optical absorption implies that our model can well explain the absorption of silicon-nanostructure thin film in the region of absorption edge.