Abstract: This article describes a welding test of two aluminum plates both about 1mm thick, a test that performed on a designed plate-welding test platform. The observation of the microstructure of the wavelike welding interface which is scanned by SEM is shown in this article as well. In order to make some explanations, a 3D numerical modeling of the collision between two welding plates during the EMW process is performed. Then the regularity of the changable vertical stress and the curve of colliding velocity are obtained and the simulation corresponds with the experimental observation. As a result, not only the colliding velocity but also the colliding position affect the colliding stress, the colliding velocity, stress and the fluidity of materials lead to the wavelike welding interface, so controlling them is the key to improve welding effect.