To cope with the demands of monitoring under high voltage and high isolation cases for traction systems of electric trains and intense magnetic field coils, a new pulse width modulation and demodulation circuitry with electromagnetic isolation is proposed. The function, performance and reliability of the circuit are discussed from the aspects of working principle, mathematical deduction, software simulation and engineering test. Application results show that, the circuit can effectively solve the problems of monitoring in cases of high voltage and high isolation, with advantages of high precision, high isolation, high response, low power consumption, stable and reliable.