A new method, the particle swarm optimization algorithm for designing the laser beam shaping system was put forward. According to the law of conservation of energy, the transformation relation of the coordinates of two intersection points between an arbitrary ray and the input and output plane can be deduced, and the coordinates of an intersection point between a ray and the output plane achieved based on this relation will be used as the target coordinates. Absolute values of differences between the target coordinates and the corresponding coordinates obtained from the ray tracing will be used as the evaluation function of transformation of energy distribution, the absolute value of the y-component of the direction vector of outgoing ray will be used as the evaluation function of the collimation, and the sum of the evaluation function of energy distribution transformation and the evaluation function of collimation was the total evaluation function of a beam shaping system. Taking this function as the fitness function of the particle swarm optimization algorithm and manipulating it to the minimum value, an aspheric lens can be gotten, which can not only convert an Gaussian intensity distribution into a uniform distribution but also approximately re-collimate the output beam. A single aspheric concave-flat lens was designed by using this method, it can transform red laser beams with a spot radius of 30 mm and a Gaussian intensity profile into flat-topped beams with a spot radius of 36 mm, and output beams can be re-collimated.