Abstract: The angular rate saturation which the angular rate of spacecraft goes beyond the measurement range of gyroscopes was researched.The countermeasure for the miscarriage of justice based on the base of fault mode analysis was presented.The threshold of parity equation from the engineering point of view was also comfirmed and also improved parity equation was improved.The result of simulation indicates that the improved parity equation can not only detect the break fault in time but also can shorten the delay time when the slow fault such as the drift of gyroscopes is detected.