Abstract: The dispersion curve of Lamb wave transmitting in aluminum alloy plate was analyzed theoretically. A signal generator instrument and an electronics sensor were used for generating ultrasonic signals. A laser vibrometer, a digital oscilloscope and a computer were used for receiving signals and making signal processing and STFT signal processing was adopted to analyze lamb wave transmission mode. Three group of experimental results showed that there was A0 mode in a nondestructive plate, and when lamb wave transmits in an aluminum alloy plate with small hole, no extra frequency produces and mode A0 changed into mode A1, and in an aluminum alloy plate with cutting line, a new frequency appears and lamb wave mode changed from A0 to mode A0 and S0.