Aiming at such the problems existed in a certain shipborne INS as the performance degradation, reliability reduction and increasing fault rate, a solution is designed for the fault diagnosis in hardware, information available and software. Especially the equation and method for the fault diagnosis is derived when the slow drift appears in one of the four channels of main and back-up INS. The result proves that the method is used to rapidly determine where the fault is. The intelligent fault diagnosis can position the fault on a plug-in board and main components like gyro and acceleration. The application of the method has a real and far-reaching significance.