Abstract: On account of existing problems of ultrasonic inspecting concrete defect,such as low signal noise ratio(SNR) and limited penetrating depth,the method of exciting with LFM is put forward.Linear frequency modulation(LFM) signal with the characteristics of long time series and wide band width is excited at transmitting terminal,and compressed signal with narrower main lobe and higher SNR will be obtained at receiving terminal through compression method.The effect of transducer's bandwidth is analyzed.The results of numerical calculation show that good compressed results of echo will be obtained if there is much noise.Testing results show that pulse compression method can improve signal-noise ratio.