Measurement and diagnosis of vehicle interior noise fluctuation when the vehicle is running at high speed are carried out in this paper. The result of order analysis of the interior noise showed that there are 2.00 order and 2.24 order noise in the vehicle during overtaking process. The noise frequency of these two orders is close to each other， which is apt to cause subjective complaints caused by modulation phenomenon. This conclusion is proved by the combination filter playback with subjective evaluation of the interior noise with a high fidelity sound playback system. The test results of acoustic vibration transfer function show that the vehicle body is sensitive in the frequency range of modulation phenomena and it is apt to produce interior noise under external excitation. The speed ratio analysis of transmission system show that the 2.24 order noise comes from the higher-order excitation of tire non-uniformity. The modulation of 2.00 order noise originated from the engine excitation and the 2.24 order noise generated by tire excitation are the root causes of fluctuation feeling of interior noise at high speed. When using tires with good uniformity， the interior noise modulation disappears， the noise fluctuation decreases significantly， and the subjective rate is greatly improved.