Abstract: In order to study the aerodynamic noise generated by a car running at high speed, the mechanism of aerodynamic noise generation was analyzed. The transient flow structure around a car model and the pulse pressure were calculated with large eddy simulation(LES). The results show that airflow breaks behind Apillar, forming a separated flow region and reattached flow region on the side window. The eddy comes into being and breaks off continuously in the separated region, producing pressure fluctuations which is the source of aerodynamic noise. Pressure fluctuations on the surface of the car model is great at low frequency and diminishs with the frequency increasing. The estimated results show that the pressure fluctuation increases with approximately the fourth power of car speed, that is, when the speed of a car is double, the level of pressure fluctuation will increase to 12 dB, which agrees fairly with the experimental result.