Abstract: Study on weathered rocks is important in engi neering geology. At prese nt, longitudinal wave velocity is widely applied to reflect the weathering and c rushing degree of rocks. However, longitudinal wave velocity is only one of the parameters of rocks, and simple velocity testing will lose much useful informati on. 　　Now, some scholars are studying rock mechanics properties by other acoustic para meters, especially the acoustic attenuation index. Their important conclusion is that acoustic attenuation is more sensitive than wave velocity when reflecting the discontinuity and weathering crushing features of rock mass. 　　Based on the viscosity acoustic absorption theory, the amplitude fades according to the law of exponential function of frequency and viscosity in the course of wave propagation. The higher the frequency is, the faster the amplitude fades. H igh frequency composition of wave varies with the absorption in rock materials. Degree of frequency abundance in receiving wave is closely related to properties of rock mechanics. In our experiment, the wave shape recorded by the high preci sion multi-parameter analyzer (digital model DB4). Computer is used to analyze w ave shape data with FFT in order to convert time domain signal into frequency do main signal. Then receiving spectrum and frequency response function is gained, and their frequency distribution features are applied in the study on rock mechanics. The method avoids complexity of acoustic attenuation coefficient test. Mor eover, it can automatically and continuously collect test data. 　　Study has demonstrated that the high weathered and crushed samples have only low frequency composition, and the fresh and hard ones have abundant high frequency composition. 　　Given the same sample size and proper frequency, geometrical and diffusio n attenuation can be effectively controlled, then internal friction absorption can be regarded as the main cause of test result.