Exposed to strong laser in air, the photoluminescence (PL) peak position of porous silicon (PS) shifts first to blue side followed by a pinning at a constant wave-length, while PL intensity degrades rapidly first, then increases slowly. Fourier transform infrared absorption spectra show a continuous enhancement of vibrations related to oxygen and a continuous attenuation of vibrations related to hydrogen with exposure time. A new physical model on light emission of PS, which says that incident light excites electron-hole pairs in nanoscale silicon units (NSUs) and the electron-hole pairs diffuse outwards and recombine through the luminescence centers (LCs) outside NSUs, can explain the experimental results successfully. We think the main LCs are hydrides, such as polysilanes or Si-H bonds, in as-prepared PS samples but transfer tinder laser illumination to be point defects or impurities in SiO2 layers which cover the NSUs.