The model of nMOSFET threshold voltage was established based on study of voltage distribution in strained Si film,which was grown on relaxed SiGe virtual substrate.The model was analyzed with reasonable parameters, and the dependence of threshold voltage on Ge fraction and channel doping was revealed. The dependence of threshold voltage shift on Ge fraction was also obtained. The relationship between threshold voltage and strained Si layer thickness and doping was studied. The results indicates, the threshold voltage increases with increasing doping concentrations of relaxed SiGe layer, decreases with increasing Ge fraction of relaxed SiGe layer, and increases with increasing strained Si layer thickness. This threshold voltage model provides valuable reference to the strained-Si device design.