The contact system is a key component of the air circuit breaker(ACB),and the degree of contact wear is a key parameter reflecting the lifespan of ACB,but it is difficult to measure directly online.A method is proposed to indirectly measure the degree of contact wear by directly measuring the spindle rotation speed.Firstly,using the principle of electromagnetic induction the sensors is designed to measure the rotational speed of the spindle.Then,the finite element electromagnetic simulation is used to optimize the specific parameters of the sensor.Finally,a model prototype is processed and the experimental comparisons are conducted on the different levels of contact wear.The experimental results verified the effectiveness of the designed sensor.