A system and method for judging interior looseness of a power transformer. The system for judging and initially positioning interior looseness of a power transformer comprises a signal conditioning circuit, a data collection device and a data analysis module that are sequentially connected, and further comprises three vibration sensors. The three vibration sensors are connected to the data collection device through the signal conditioning circuit, and measure vibration signals of three phases of the power transformer correspondingly. The vibration signals are amplified, filtered and denoised by the signal conditioning circuit and then transmitted to the data collection device. The data collection device transmits the collected vibration signals to the data analysis module. The data analysis module stores and processes the vibration signals and carries out fault judgment. By means of the solution, the looseness fault of the transformer is judged through 100 Hz signals in the vibration signals according to the characteristic of the change of the vibration signals along with the looseness degree.