Abstract: Improved BP algorithm——momentum factor-adapted learning rate algorithm was cited. Based on BP algorithm, the adaptability was realized by momentum factor adjust and the learning rate to improve the learning speed and stabilization. By useing the algorithm in multilayer forward fuzzy neural networks of serial structure, the fuzzy mapping relation and the fuzzy malfunction diagnosis of the object were obtained. When the fuzzy neural network was used in malfunction diagnosis, the inspected malfunction sign connected with the import layer of networks to put the import vector into network. Then the fuzzy subjection limit vectors of the defined sign which belongs to the malfunction sign by dealing with fuzzy were achieved. By counting with the neural networks forward, the fuzzy subjection vectors of the malfunction cause were gained. The type of malfunction cause was analyzed and ensured, and this fuzzy neural network in diagnosing the malfunction of air still press axletree has been applied. The result proves that the method is effective.