In hardware realtime operating systems, the CPU utilization ratio is an important indicator of system performance. If a task occupies the entire CPU, others will not continue, which can induce a disastrous consequence of system performance. By analyzing the characteristics of software running in realtime operating systems, a certain software faulttolerance strategy must be used to enhance system reliability and faulttolerance. In the μC/ OSⅡ realtime operating system, tasks in the flight control software are realtime monitored. Firstly, the calculation method of CPU utilization ratio is given and the CPU monitoring period is proposed reasonably. Secondly, a fault detection algorithm for abnormal CPU utilization ratio is presented. By dealing with the faults, the system's fault tolerance ability can be improved. Finally, the flight control software is designed in the embedded MPC5674 flight control computer to validate the ability and effectiveness of the four methods’ handling abnormal CPU utilization ratio. Simulation results show that software faulttolerance based on monitoring CPU utilization ratio can enhance the system reliability and faulttolerance ability effectively.