This paper describes an automatic online detecting system. In the system, digital image processing technology is used to preprocess X-ray images of the products, and neural network algorithm is applied to diagnose faults. The fault recognition model adopts an improved back-propagating neural network, which is trained by a series of standard X-ray images of correctly assembled products. The detecting system combines digital radiography technology with digital image processing, and applies the back-propagating neural network algorithm in the fault recognition process. The system improves the speed and reliability of fault detection and has application in the field of industrial nondestructive detection.