Abstract: Acoustic response of three kinds of plastic was studied by contrast test block, and the respective longitudinal wave velocity and attenuation coefficient μ were measured. The k value and frontier of probe were measured by longitudinal wave angle probe, and it has high precision. It was found that defect location was accurately positioned and its length was rather precisely measured through simulation UT for test block. Direct longitudinal wave method was priority to adopt within detection of hot-melt welding joint. But when the thinness was thinner and with detecting condition limit, primary reflection wave can also be used.