In order to measure and study the contact impedance of the contact interface between the central rail and the electromagnetic launching system, a low impedance measurement system at low frequency is developed. The system measures the vector voltage at both ends of the impedance to be measured and the standard impedance separately by impedance transformation method. The real and imaginary parts of the vector voltage are separated by phase sensitive detection method based on free axis. The magnitude of the impedance to be measured is calculated according to the relationship between the standard impedance voltage and the impedance voltage to be measured. The system uses STM32 controller as signal generator and controller, adopts Kelvin four-wire method to reduce the influence of lead resistance and contact resistance, and uses software compensation method to reduce measurement error. The measurement results show that the measuring system can work normally within 10 kHz and has high accuracy in measuring resistance within 100 Ω.