Ultrafast relaxation process of photoexcited charge carriers in semiconductor nanoparticles is modeled with the analysis of state levels structure. Several parameters, that affect this process, are discussed. The result shows that, with the increasing of excited intensity or decreasing of surface state density, the electron saturation of the surface state would cause the electron build-up of conduction state and lead to a longer life time. The relaxation of deep trapped electrons is the main limit of response time for nanoparticles. The model is used to analyze pump-probe experiment, showing potential use in experimental analysis.