According to the characteristics of Lambertian source, an approximate Lambertian source function was defined, an optical model of LED lens was designed, and then a LED sectional curve equation was obtained. Some relative data and modified data were obtained through solving the equation above by Runge-kutta method and a polynomial fitting by MATLAB. The expecting data and simulated effect were extracted from LED approximate Lambertian source encapsulation model by Tracepro. Finally, a simplified designation method for LED approximate Lambertian source encapsulation model was proposed.