In order to automatically adjust the positive buoyancy of an AUV (Autonomous Underwater Vehicle) under marine environment, the invention discloses a buoyancy adjusting method for the AUV, so that the AUV can steadily sail and operate under the marine environment. The method comprises the following steps of estimating the present positive buoyancy state of the AUV; determining the reduced positive buoyancy of the AUV in the buoyancy adjusting process according to actual conditions; in order to reduce the positive buoyancy, calculating the discharge water volume of a buoyancy control device on the AUV. According to the buoyancy adjusting method, the positive buoyancy is online adjusted to a proper sailing state by the AUV and does not need to be tested by multiple times of diving, thus the method is simple and widely applied.