To effectively improve the space usage of optical systems for the light-emitting diode (LED), a freeform surface optical lens with a negative focal length transmission structure was designed. The optical lens was divided into two parts, including the transmission collimating module and reflection collimating module. The collimating lens was designed using geometrical optics, Snell’s law, and the law of energy conservation. The two-dimensional curve data of the freeform surface for the collimating module was obtained through iterative calculations with Matlab software, which was then imported into Solidwork software and rotated 360° around the central axis to obtain the three-dimensional solid model. Finally, the model was introduced into the optical simulation software, Lighttools, for light tracing and optimized according to the spot situated on the receiving surface. The optimized collimating lens was simulated with an optical energy utilization rate of 88.45% and a beam angle of ±2.23°. The results show that using a negative focal length transmission structure design can effectively reduce the volume of the collimating lens by 63.13% and improve the ability to control LED beams compared with the conventional single transmission freeform surface collimating lens.