Abstract: Under the condition of preserving zero-crossing positions of the original waveform, a string description approach which approximates the original waveform with several kinds of line segments as primitives is presented. The number of primitives can easily be adjusted according to waveform complexity and the required accuracy. In general, a string representation consisted of only three kinds of primitives will contain a great deal of information of the original waveform and can be used in the syntactic approach for waveform recognition. The experiments on seismic signals demonstrate the effectiveness of this approach.