A combination design method of freeform surface of multidirectional parallel beam splitting lens is proposed, which can be applied to the light emitting diode (LED) light source of generalized Lambertian distribution to realize any multidirectional parallel beam splitting. A lens is designed for the rainfall sensor, which can increase the number of outgoing ray paths without adding the light source. The designed lens has such advantages as small volume and high luminous efficiency, which can help to increase the optical detection of the sensor area and provide convenience to the design and optimization of related photoelectricity devices. Theoretical calculation and modeling simulation further verify the feasibility and validity of this plan.