Abstract: The reliability of the material nonlinearity coefficient measurement is affected by the performance of nonlinear ultrasonic testing system performance directly. In nonlinear ultrasonic testing, the amplitude of harmonic signal is usually smaller than the base signal, and difficult to be detected. This topic analyses how to extract the second harmonic signal from the test system needs,the corresponding filter system is developed,and the design of high-order active filter system is given. Through the study of the simulation of the system, the simulation results confirmed that the system has realized the function of extract small amplitude from the large amplitude signals. At the same time, it can also suppress some nonlinear factors produced by the instrument itself.