According to the optical structure of LED device,a mathematical model of LED encapsulation is established and the corresponding equations are given.Taking advantage of Matlab to solve the final differential equation, we obtained some data on the sectional curve of LED encapsulation of homogeneously rectangular irradiance at x,y directions by the fourth order RungeKutta method.Then after modeling and simulation by some related software,a 3D model of first lens was obtained which for LED homogeneously rectangular light field.The method has strong function and is suitable for extensive dissemination in the actual engineering applications.