The ZnS thin film is deposited on a porous Si substrate by using a pulsed laser deposition method. The crystalline structure, surface morphology and optical properties of the ZnS thin film are studied respectively by using a X-ray diffraction instrument, a scanning electron microscopy and a fluorescence spectrophotometer. The results show that the ZnS film has a crystalline structure in cubic phase and is grown in a prefered orientation along the β-ZnS (111) direction. After annealing in vacuum at 300℃ for 30min, the X-ray diffraction peak intensity of the ZnS film is increased and its surface becomes rough. In the visible spectral region, the film has an average transmittance of 80\%. So, it is suitable for the buffer layer of solar cells, infrared antireflection coating, infrared windows and domes etc.. In the photoluminescence spectra of the annealed ZnS/porous Si composite system, besides the blue light emission of the ZnS film at the high energy end and the red light emission of the porous Si at the low energy end, there appears a new green light emission at about 550nm. This is attributed to the defect-center luminescence generated by the defect energy level formed in the annealing process of the ZnS film. According to the tricolor overlay principle, the blue and green light of the ZnS is combined with the red light of the porous Si. As a result, more intensive white light is generated from the ZnS/porous Si composite system. This opens a new way for the realization of solid white light emitted devices.