This study made an effort to analyze the failure features of sandwich plates composed by composite faceplates and foam cores under impact. Drop weight tests were carried out involving different impact energy, foam density and stacking sequences. Ultrasonic microscope and cross section observation was conducted to reveal the failure details within the structure. The delamination among the top faceplate was found to follow the debonding law of composite laminates. The penetration of the top plate will change the shape of delamination region and stop the expansion of the delamination area as the impact energy increases. Sandwich structure with a hard core was proved to be more vulnerable to delamination than the one with a soft core, but this difference became negligible when penetration took place. Crush region and fracture region were located in the structure respectively, which was helpful for strength estimation and repair works. A new numerical model based on user-defined material subroutine was proposed for sandwich structures under impact. The model has the advantage of reproducing the failure features correctly and was proved to be effective.