Nuclear power plants typically extend the operating period of units at the end of life to improve economic performance and operational flexibility. At the end of life, the boron concentration becomes too low, requiring a reduction in coolant temperature and core power to introduce positive reactivity. When maintaining the average temperature automatically, the slope of the reference average temperature function needs to be gradually adjusted according to the actual burnup. As the slope of the coolant temperature reference decreases, the response capability of the turbine bypass control system deteriorates. The operating margin of some protection signals significantly decreases in load rejection condition, which increases the probability of reactor protection system response. This paper discusses the impact of stretch out operation on the operating margin of protection signals, and presents the logical optimization design and parameter verification of the turbine bypass control system to improve the operating margin, providing insights for improving the operating margin of protection signals during transient conditions in stretch out operation of nuclear power plants.