Abstract: Aiming at the delayed activation of the source range channel in nuclear power plant ex-core nuclear instrumentation system, this study clarifies fault inducements and proposes optimization measures. By sorting out the structure of intermediate range detectors, signal flows and inter-range interlock logic, detector faults, compensation voltage and current-to-power conversion coefficients are identified as major influencing factors. On the basis of existing countermeasures including compensation voltage adjustment, parameter modification and logic degradation, an RNN time-series model is introduced to predict source-range count rate for assisting activation decision-making. Validated by measured data from Outages X04 and X05, the model prediction deviation is less than 1100 CPS. This research provides theoretical basis and engineering references for fault handling and stable operation of ex-core nuclear instrumentation systems, and helps improve unit operation efficiency during the power-down phase.