Abstract: In hard real-time system,overtime of task running can induce a disastrous consequence of system performances．To enhance reliability and fault-tolerant ability of system,some software fault-tolerance strategy must be used．CPU utilization ratio of system is a key characteristic,which tells whether system is normal or not．Time feature and task state can be shown by CPU utilization ratio．Because of characteristic of CPU utilization ratio and demand of fault-tolerance,machine cycle was selected as measurement baseline.There are great differences when different monitor cycle is selected．Conditions of judging CPU utilization ratio anomaly in system were presented．Four measures that refer to structural redundancy and time redundancy were designed to handle this case．The simulation shows that software fault-tolerance based on monitoring of CPU utilization ratio can enhance reliability and fault-tolerant ability of system effectively.

Key words: Real-time system,Software fault-tolerance,CPU utilization ratio,Exception handling