The invention discloses a collimation lens and a design method thereof. The collimation lens has a rotation symmetrical structure, a light source center is arranged on a rotation symmetrical axis, and the lens comprises an incident plane, a first reflection plane, a second reflection plane and an emitting plane. The first reflection plane is disposed between the incident plane and the second reflection plane, light emitted from a light source passes through three parts, the first part of light is emitted from the emitting plane at the top portion of the lens without passing through any optical media; the second part of light is refracted through the incident plane, then is totally reflected through the emitting surface, then is incident to the second reflection plane and then is emitted from the emitting plane after being reflected by the second reflection plane; and the third part of light goes into a first emitting plane after being refracted by the incident plane and then is emitted from the emitting plane after being totally reflected by the first emitting plane. The lens can solve the defects of quite large volume and weight of a conventional lens in the prior art and reduces the manufacturing cost of the lens.