According to the speed-test principle of high-speed motion object, a method has been proposed which uses electromagnetic sensor. On the basis of the test requirements, analyzing the principle of electromagnetic induction, and designing an electromagnetic sensor which can meet the requirement. The magnetic dipole model of magnetic projectile is established in the process of analyzing to analyze the electromotive force produced by induction coil when crossing the electromagnetic sensor in the test tube and the changing of motion object caused by the volume changing of gas. At last verifying the feasibility of electromagnetic sensor by means of simulated and real experiment, and analyzing factors influencing the precision of testing.