System and method of detecting intrusion into vehicle

A system and a method of detecting an intrusion into a vehicle includes: a transmitter that transmits an ultrasonic wave signal into the vehicle when the system enters an intrusion-into-vehicle monitoring mode; a receiver that receives an ultrasonic wave signal reflected by an object inside the vehicle; and a controller that generates an intrusion warning when it is detected that a frequency level of the ultrasonic wave signal flowing in the receiver is changed, determines that a frequency synthesis is made to the ultrasonic wave signal by an external ultrasonic wave device when a changed frequency waveform having a predetermined magnitude is continuously received by the receiver, and defers generation of the intrusion warning.