Abstract: The principle of the spectral analysis of actual full waveforms from the Long Spacing Sonic wave (LSS), as well as the main factors affecting this analysis, are discussed and the formula for the estimation of frequency errors is derived also. From these, the variations in the spetrum of each period on the waveforms are analysed and the primary and secondary spectrum values of the compressional, shear and Stoneley waves of some common rocks are given. The continuous primary spectrum of these three types of waves are given The most important is that we work out DASP curve which has been proved to be effective to indicate gas-bearing layers. All results in this paper are based on actual logging waveforms, and all tables and figures are taking from actual field cases.