Abstract: To resolve the problem of low locating rate in fault diagnosis, efforts were made by using preceding fault information fully in lot assure tests. By studying the failure rules of air-bone radio equipment in lot assure tests, the fault probability under extreme-small-sample conditions was estimated using Bayesian method. An approach for direct fault detecting in next step was established by using the fault probabilities of different modules. Three methods of direction were adopted to simulate those probabilities, the results showed that the strategy of the present paper can effectively improve the rate of locating faults in only one detecting progress and reduce fault finding time.