Based on the law of conservation of energy and the mesh generation method, a lens with large view angle for ultra-thin direct-down LED panel light is proposed. The illumination distribution of the target plane of the Lambertian type LED at a height of 96 mm is used as the desired illumination distribution to optimize the feedback of the large angle LED lens, and form a light field with a desired illumination distribution on a target plane with a height of 25 mm. To investigate the influence of the large angle LED lens after optimization on the thickness reduction degree of the straight down LED flat lamp, the height of the target surface is set to be 20, 25, 30 mm respectively, and then the spacing of the square LED arrays is set to be 96, 82, 75 mm respectively. The software LightTools is used to simulate and analyze the uniformity of the target illumination of the direct LED flat lamp. The results show that when the target plane height is 25 mm and the pitch of the square LED array is 82 mm, the illumination uniformity of the target plane is the best. And the value (the minimum illumination is divided by the maximum illuminance) is 0.8756, which meets the national technical requirements for the application of LED indoor lighting.