Abstract: To improve the precision of time span measurement during measuring sonic velocity, we carried out spectrum analysis on signals of ultrasonic sound velocity got by commonly pulse echo method, and found that the signals were distorted due to beam spread. The distortions took some effect on the measure of time span between echoes. The use of sine wave from function generator as exciting source, and with double probes to excite and receive ultrasonic echoes, can improve accuracy of time span measurement, and reduce harmful effects to instruments.