A numerical study is made of the operation strategy of a once-through steam generator considering degradation in the thermal-hydraulic performance. The performance of the steam generator is gradually degraded as it experiences fouling, clogging, and plugging during its lifetime. To maintain the thermal power of the steam generator, therefore, its operating condition should be properly changed according to the degradation rate. Emphasis is given to the operation strategy of the steam generator to transport a constant thermal power regardless of the degradation rate. Comprehensive numerical solutions to the governing equations are acquired. It is found that a constant thermal power can be maintained by properly adjusting the secondary coolant outlet pressure with a variation of the superheat degree and secondary coolant pressure drop. A constant thermal power operation curve is obtained, and the thermal-hydraulic performance of the steam generator is analyzed according to the degradation rate in terms of the degree of superheat, secondary coolant pressure drop, temperature distribution, and quality distribution in the tube.