A rain sensor of a vehicle and a control method thereof are provided to solve a problem that a measurement result of rainfall varies according to a glass specification of a windshield even under the same rainfall condition by tuning sensitivity of the rain sensor in accordance with the glass specification of the windshield. For this, a method of determining the glass specification of the rain sensor includes determining the glass specification of the windshield from a light-receiving signal obtained by receiving light reflected by the windshield of the vehicle, and eliminating a deviation between light-receiving signals by glass specification, which is caused by a difference in the glass specification, by applying parameter tuning according to a glass specification to the light-receiving signal.