Boron-coated proportional counter, operating long-term in the high-temperature, high-humidity, and high-flux neutron radiation environment outside the reactor, is susceptible to changes in their design parameters. This can lead to degradation of key performance metrics, affecting the monitoring accuracy of neutron fluence rate. Based on field failure data from nuclear power plants, this study identifies two typical failure modes -- quenching gas (CO2) consumption and seal failure -- and establishes corresponding degradation models using Geant4 and Garfield++ multi-physics coupling simulations. The simulation results indicate: (1) CO2 consumption leads to a shortened plateau length and increased plateau slope. The recommended operating voltage decreases, and the signal pulse width decreases. The discrimination characteristic curve shifts towards higher thresholds, exhibiting a “low voltage - high discrimination” characteristic. (2) Seal failure (air infiltration) resulting from seal failure causes a shortened plateau length and increased plateau slope. The recommended operating voltage increases, and the signal pulse width increases. The discrimination characteristic curve shifts towards lower thresholds, exhibiting a “high voltage - low discrimination” characteristic. This research provides a theoretical basis for the fault diagnosis and life prediction of proportional counter.