Abstract: The stiffened plate is a commonly used structure in spacecraft. In order to develop the onboard monitoring system based on acoustic emission technology, the wave propagation in a stiffened plate and the source locating method are important issues. In this paper, the speed of waves in different directions in a stiffened plate is measured by lead break tests. The effect of different dimensions of stiffeners is studied by FEM. Test and simulation results show that the stiffeners have but little effect on the extensional wave speed, but they cause an attenuation of the first peak amplitude. Assuming that the signal propagation velocity in the stiffened plate is a constant, the source can be located by the optimization of seeking the minimum of a function. From the locating tests of the lead break source and the hypervelocity impact source that were carried out on the stiffened plate, good results with small errors are obtained.