The precision of the lens construction by equi-flux map method should be improved. Firstly, the core of this method is analysed. The result shows that the geometric meaning of this method is the same as the Euler method in numerical method in solving differential equation. Then, based on Runge-Kutta method, the method of lens construction of equi-flux map is improved. Two lenses which have 60° beam angle are designed in the same condition. These two lenses are simulated in the optical design software. The result of ray tracing shows that the illuminance distribution of the lens designed in the improved method is closer to the prescribed distribution. The improvement of the precision of the lens construction is achieved by this method.