Induction-type coil target is a common used zone-block device for measuring projectile’s velocity. The magnetized projectile is simplified as the point magnetic dipole, and then the physical model of magnetized projectile passing through the coil target is established, based on these the mathematic model of inductive electromotive force when the magnetized projectile flying through the coil target in a certain speed is deduced with the electromagnetic theory; According to the model, the characteristic parameters such as the zero-crossing point, the slope of zero-crossing point and the extreme value point are discussed, finally the principle and the influence factors of velocity measurement by single-coil target and paired- coil targets are described. All these above provide some foundation and advices for designing and using induction-type coil target.