An automobile detector is designed based on tunneling magnetoresistance (TMR) sensor. Key specifications of the automobile detector are determined by theoretical analysis and simulation. The characteristics of the magnetic anomaly signals caused by automobiles are measured and analyzed, and the circuit is developed to detect the signals. Performance of the detector is calibrated and tested, including linearity, noise level, bandwidth, detecting distance, and detection rate. Test results show that the farthest response distance of the detector is up to 4 m, and the detection rate is 100% within 3.5 m. The automobile detector has advantages in precision, linearity, dimensions and power consumption, which can be applied into highly sensitive automobile measurement and detection.