Abstract: Applying classical wave theory,a concept of equivalent elastic modulus on propagation speed modification of guided wave through“variable cross-section”of metal bar embeded layer was put forward.The transmission mechanism of guided wave at joining-section of metal bars and embeded anchor was analyzed theoretically,and an idea of low-frequency impulse coupled whth guided wave propagating through joining-section was presented.These conceptions might be used for interpreting “time-lag” of signal and leveled phenomena of waveform-signals that were produced on time-domain figures in application of guided wave to operate nondestructive test.