Based on the principle of ultra-short channel MOSFET, a definite solution of potential is proposed by introducing two rectangular sources between the insulated gate and the space-charge region. By using the semi-analytical method and the spectral method, the 2D semi-analytical solution has been obtained for the first time as faras we know. The solution is a special function for the infinite series expressions. The most advantage of this model is that it can not only be calculated directly without numerical analysis but also keep the same accuracy as that of numerical solution. In addition, this model, which can be directly used in circuit simulation, has the characteristics that in its expression there is no adapter parameter with small calculating amount. The potential, surface potential and threshold of 45—22 nm MOSFET have been calculated in the frame of this model. It is shown that the calculated results are identical with Medici.