The invention provides a high-security computer system based on lock-step and monitoring and a design method thereof. When the processor performs a read operation, the dual processors respectively read data from the same address of the two memories and compare them. If the comparison is correct, the data is sent to the corresponding processor. If the comparison is wrong, the fault handling operation is performed; the comparison module for monitoring and lockstep is set up, and the checkpoint output is added to the load. When the checkpoint arrives, the cross comparison is performed. If the comparison is correct, the system is based on operation; if the comparison is incorrect, the troubleshooting operation is performed. The invention realizes fault detection and location at the instruction level, greatly reduces system faults caused by random faults in the memory, improves the detection rate of the dual-processor common-mode faults of the lockstep module, and further improves the safety of the entire system, effectively It solves the problem that the running speed of dissimilar processors does not match, and reduces the busy waiting time of the monitoring unit.