During the operation of a reactor, continuously monitoring the primary loop cooling water to detect potential fuel cladding failures is essential. To address the data latency caused by the widely used sampling methods in nuclear power plants and the frequent exposure of sampling personnel to radiation, this study investigates an online measurement method utilizing gamma spectroscopy. We propose an online gamma spectrum analysis method based on MCNP simulations and a self-developed GEAP program. By constructing a detector-pipeline geometric model and a dual nuclide library for chemical sampling and online monitoring, we conduct simulations for nuclide qualitative and quantitative identification using the independently developed GEAP gamma spectrum analysis program. The accuracy of the quantitative and qualitative analysis conducted by the GEAP program was validated, showing that the discrepancy between computed values and theoretical values is less than 5%. This research provides a theoretical basis for real-time monitoring.