The efficient infrared-upconversion luminescence generation, which was considered as an enhanced third-order nonlinear optical process by our primary study, was used as a probe of the bulk symmetry properties and mechanism of light-emitting porous silicon layer (PSL). It was found that the crystalline anisotropy of crystal silicon is almost remained for PSL. The observation of laser-induced decreasing of infrared-upconversion signal demonstrated that the presence of hydrogen in the PSL, is essential to the luminescence efficency.