Abstract: Pill velocity measurement radar inside cavity of electromagnetic rail gun usually works in the environment of strong magnetic field. Based on the analysis of basic structure and principle of electromagnetic trail gun and pill velocity measurement doppler radar, pill velocity measurement radar EMC issue under strong electromagnetic environment is discussed, included electromagnetic radiation feature of electromagnetic trail gun, technical principle of electromagnetic shield and shield request or design consideration. It designed a suit of feasible velocity test system of electromagnetic rail gun inside cavity by means of velocity measure doppler radar．The system could be adopted to measure the pill velocity of electromagnetic rail gun inside cavity in real time，accuracy and auto-measurement，it solved the emergent puzzle in the electromagnetic rail gun research．