The invention discloses a method for measuring an oil level of a transformer oil conservator based on ultrasonic detection. The method adopts an ultrasonic probe, a signal acquisition module, a signalprocessing module and a display module. During measurement, the ultrasonic probe is attached to the outer wall of the oil conservator and moves up and down in the height direction of the oil conservator, ultrasonic longitudinal waves emitted from the ultrasonic probe can penetrate through the metal wall thickness of the oil conservator and the insulating oil in the oil conservator, the ultrasoniclongitudinal waves generate reflection signals on penetrated different medium interfaces, and the reflection signals are received by means of the signal acquisition module. The signal processing module controls the display module to indicate a number of measured reflection signal times according to a number of reflection signal times measured at a position of the oil conservator at the corresponding height. The oil level of the oil conservator is positioned between the heights of the oil conservator at which a primary reflection signal and a secondary reflection signal can be received. The method can be used for detecting the internal oil level of the transformer oil conservator rapidly and precisely under the condition that the transformer is not shut down, is stable in measurement and has high repeatability.