Color light signal is an important part of China railway signal outdoor equipment, but the signal light source in bad weather conditions due to atmospheric sol scattering and absorption will lead to the inconformity between the output light and the actual identification color of drivers. To ensure that the light source can transmit the correct traffic signal, on the basis of the existing LED railway signal, an optimal design scheme of signal lens structure based on free-form surface was proposed. The freeform surfaces of the transmission and reflection parts of the lens were iteratively calculated and numerically fitted according to Snell's law. The calculated lens bus was imported into SolidWorks 3D modelling software to complete the 3D lens modelling, and the results of LED lamp beads and lamp beads embedded in the lens were simulated and compared with TracePro optical software. The simulation results show that compared with the direct emission of the LED light source, the maximum of the total luminous flux on the receiving surface of the LED light bead embedded in the free-form lens is increased by nearly 500 times, and the beam dispersion angle of the LED light source is reduced from 90° to 6°. On this basis, the design of the lamp plate for the railway signal field was further developed and compared to the honeycomb lens. The optical analysis and experimental results demonstrate that the free-form lens structure provides better light distribution uniformity than the honeycomb lens. Furthermore, the new LED railway signal meets the requirements of TB/T 2353-2018 "The Luminous Intensity of Railway Signal Light", which can realize the light color of light source to be correctly transmitted to the driver's line of sight after atmospheric sol scattering and absorption, and further ensure the driving safety.