The invention patent relates to a collimating lens structure with a freeform surface and a design method thereof. The collimating lens system is designed by using a freeform surface method to design an integrally formed collimating lens, where the design method is based on geometric analysis and calculation on the tangential vector for each optical surface. Then the coordinates of points on each optical surface is calculated by an approximation or exact method, and a three-dimensional model construction software is used to construct an ellipsoidal optical collimating lens. Through the above design, the ellipsoidal optical collimating lens can be applied to an LED light source such that the light is projected to a disc detecting surface of 35-m diameter and 200-m away, and the angle between the light source and the detecting surface is about ±5ﾟ that the optical collimation efficiency of 88％ is achieved at least. Meanwhile, the problem of the conventional collimating lens, which is composed with complex lenses and leads to a waste of space and poor collimation efficiency, can be solved, greatly improving the economic efficiency of the collimating lens.