Aiming at the speed measuring and location error problems caused by vehicle vibration, a new large gap and area-variable eddy current sensor which uses first difference is designed. The sensor is composed of test coils, excitation circuit, power amplifier circuit, signal detection circuit, demodulation circuit, differential comparison circuit and sampling circuit. The excited signals produced by excitation circuit are amplified, then provided for the test coils and the demodulator. By the appropriate design of the test coils, the signal detection circuit, and the posterior processing circuit, the function of speed detection and relative location can be achieved quickly, exactly and reliability. The availability of the new sensor is verified by ground simulated experiment. The test results show that the new sensor can detect the position of the sleeper accurately, a 5 cm detecting distance is achieved and the measurement results will not be influenced by vehicle vibration.