Based on the analysis of Poly-Si1-xGex gate work function and by solving Poisson equation, the models of vertical electric field and potential distribution in strained Si NMOSFET with Poly-Si1-xGex gate are obtained; threshold voltage model and the gate depletion thickness and it's normalization model are established in strained Si NMOSFET based on the above results, with the gate depletion effect of Poly-Si1-xGex taken into account. Then the influences of device geometrical and physical parameters of device especially the Ge fraction on Poly-Si1-xGex gate depletion thickness are investigated. Furthermore, the effect of gate depletion thickness on threshold voltage is analyzed. It shows that the poly depletion thickness decreases with the increases of Ge fraction and gate doping concentration, while it increases with the increase of substrate doping concentration. Furthermore, the threshold voltage increases with the increase of gate depletion thickness. The results can provide theoretical references to the design of strained Si devices.