With the development of economy, the car ownership of car users continuously increase in recent years, in order to improve the competition of cars, car manufacturers keep adopting novel technologies, novel materials and novel processes to improve the power, the exhaust, the safety and other technical properties of cars, and simultaneously, the after-sales services of cars are more and more concerned about by people. However, now China's automobile service enterprises have not high quality and unreasonable structure, and 'many, small, poor and messy' are still the main problems of automobile repairing enterprises. The invention relates to an automobile fault diagnosis system based on ontology and a mobile network, which realizes the sharing of various resources required by automobile repairing with the advantages of a body, remotely communicates through mobile communication, internet and other methods so as to obtain the fault information of cars, so that the fault cars are finally quickly and efficiently repaired. On the one hand, the system can be promoted by large automobile manufacturers as an after-sales service system. On the other hand, the system can be also applied to the claims demonstration of insurance companies.