Wear is one of the main factors of machine failure, wear failure holds more than 70% of mechanical parts failure. Debris carries a lot of information of running devices. Inductive sensor is wildly used in online detection of oil debris. But most of the former studies are based on the single-layer solenoid model of the inductive sensor. In this paper, the basic principle of the multilayer inductive wear debris sensor is introduced. According to the theory of electromagnetic field the magnetic field along the axis of sensor is presented. Then, the output characteristic model of the debris sensor is proposed. Based on the model, the output characteristic of the inductive debris sensor is numerically simulated with different sensor's design parameters by using matlab, the theoretic model and numerical simulation can provide more accurate guideline to the parameter optimization for the design of the inductive debris sensor.