Abstract: When measuring the imperfections of forging pieces at site,the measurement of imperfection extended length is an important judgment parameter for many standards.Because of the influence of sound beam diameter of ultrasonic wave,there shall be a big difference between the gage length and actual length of imperfections,which will exert big influence to the judgment of measurement.The article is to use the dynamic oscillogram of imperfections to check if the imperfections with the punctulation imperfection property and imperfection extensibility property,and use imperfection equivalent value and 6dB measured value to judge the extended length of imperfections,so as to decrease the erroneous judgment and to improve the credibility of inspection result and decrease the economic loss.