The invention discloses a system and a method for diagnosing and preliminarily positioning loosening faults of an iron core of a power transformer, and belongs to the technical field of intelligent diagnosis of faults of power transformers. The system for diagnosing and preliminarily positioning faults comprises vibration sensors, a signal acquisition instrument and a computer system. The method for diagnosing and preliminarily positioning faults includes that vibration signals are measured by the three vibration sensors mounted on the top of the transformer, Fourier analysis is carried out for the signals to obtain characteristic quantities, which include a 50Hz component, a 150Hz component and a 300Hz component, of the loosening faults of the iron core, the 300Hz component is used as a main characteristic quantity, a certain position of the iron core is determined to be loosened when amplitudes of the components of the certain position are increased to certain values, and the faults can be positioned by means of comparing the differences of the three characteristic quantities of signals of each position. By the aid of the system and the method, characteristics of the loosening faults of the iron core can be accurately reflected, and the loosening faults of the iron core of the power transformer can be effectively detected.