This paper presents electromagnetic inspection of in-service train wheel online for high-speed railway using a fast demodulation algorithm. The electromagnetic simulation demonstrates the defect can be identified by the amplitude variation of the differential voltage induced in two pick-up coils. To demodulate the induced voltage, the Kalman-model-based demodulation algorithm is proposed, which can fast demodulate resultant signal of any arbitrary frequency and have higher anti-noise ability at high speed. Furthermore, the experimental platform for online electromagnetic inspection of in-service train wheel is designed. Both defect identification and speed estimation experiments are carried out. Finally, the experimental results demonstrate that the design of online electromagnetic inspection of in-service train wheel is effective and two defects can be identified by the differential induction voltage at speed over 300km/h. Furthermore, the Kalman-based demodulation has better performance in noise suppression at a higher speed compared with other traditional demodulations.