Abstract: Taking the effect of the in-plane preloading into consideration, the impact behavior of composite panels was studied. A kind of in-plane loading device was designed to apply tension, compression and shear loads. The high-speed impact test method of preloaded composite structures was proposed with the gas gun and the in-plane loading device. Then high-speed impact tests for X850/IM+ and M21C/IMA composite panels were conducted. The test results show that the in-plane preload has a significant influence on the impact response. Compared with the non-preloaded impact process, the in-plane pre-tension raises the structural bending stiffness and the residual speed, resulting in a decrease of the penetration velocity and delamination area, while the effect of the in-plane pre-compression is opposite.