The invention provides a full bridge uncontrolled rectifier fault diagnosis method based on insulation monitoring. Insulation resistance Rt of a system under normal operation conditions is measured first; the positive electrode and the negative electrode of an IMD injecting source based on a direct current injection method are connected with a neutral line and the ground respectively, and a system insulation monitoring value Rf when IMD forward injection is carried out is obtained through measurement; the direction of IMD injection signals is changed, the positive electrode and the negative electrode of the IMD injecting source are connected with the ground and the neutral line respectively, and the insulation monitoring value Rr when the IMD reverse injection is carried out can be measured; if the Rf or the Rr is not equal to the Rt, the system enters a fault diagnosis process, the real insulation resistance Rm of the system is calculated by using the collected Rf and the Rr, the Rm is then compared with the insulation resistance Rt under the normal operation conditions of the system, and whether the changing of the insulation resistance is caused by fault of a rectifier is obtained.