This study applied the concepts of the Shewhart control chart to design a pre-alarm system for the nuclear power plant control room. As a support in detecting faults, the pre-alarm system reminded the operators of a change in the system state in its early stages. Two pre-alarm types were designed to compare with the original system, and all participants were requested to monitor each simulated system under both normal and abnormal states. The tasks for the participants included shutting down the reactor, searching for procedures, monitoring system parameters and executing secondary tasks. In each trial, the task performance, mental workload and situation awareness (SA) of the participants were measured. Results indicated that participants had lower mental workload, but equal SA, when monitoring the system with either type of pre-alarm designs, and lower alarm frequency and higher secondary task performance were obtained with the pre-alarm design. Therefore, the pre-alarm system effectively assisted the operators in monitoring tasks.