We study the principle and method of obtaining cylindrical lens type directional backlight flattop beam by using a butterfly liquid crystal switch. Based on the generalized Huygens-Fresnel diffraction integral theory, we deduce the diffraction field distribution expression of beam after passing through cylindrical lens, and the butterfly liquid crystal switch unit is the light source of the beam. Placing the butterfly liquid crystal switch unit on the cube focal plane of the cylindrical lens, we simulate transverse optical field energy distribution curves at cylindrical lens with different focal lengths by MATLAB programming, and analyze the influence of butterfly parameter on the transverse optical field energy distribution. The results show that the energy uniformity of the transverse diffraction field energy firstly increases and then decreases with the reduction of the butterfly parameter, thus there exists a butterfly parameter that can maximize the energy uniformity.