It is important to detect the vehicle rear axle on-line during production process. To realize the quality detection of the rear axle, vibration diagnosis method is adopted to analyze the vibration characteristics of the rear axle and calculate characteristic frequency. The on-line detection control system of vehicle rear axle quality is designed by extracting characteristic parameters of correctly reflecting rear axle running state and applying PLC system. The system has been operated stably, reliably and efficiently through the test. It has a practical significance.