This paper mainly treats the order ratio spectrum analysis（ORSA) and its application to the realtime fault diagnosis of the car-starters. First，it elaborates the principle of the ORSA and the way of generating and the method of being restrained to the aliasing and leakage in the process of order ratio sampling. Then the paper discusses how to acquire the interested noise signals in the short working process of the car-starters with the rapid acceleration，and how to extract the information of their fault features from the signals in accordance with the ORSA. Finally it diagnoses realtimely the faults of the car-starters by means of threshold judgement.