Abstract: A mathematical model was established for the supercritical fossil-fired power system with CO₂ recompression and reheat Brayton cycles, based on which the effects of following key parameters on the cycle efficiency were analyzed through detailed calculation with programs developed on the Fortran platform, such as the split ratio of flow, the inlet and outlet pressure of compressor, inlet temperature of turbine etc. Results show that the cycle efficiency increases linearly with the temperature rise of primary and secondary working medium. Different from traditional Rankine cycles, above parameters in Brayton cycles show non-monotonic relationship with the cycle efficiency due to the features of spercritical CO₂ physical properties and the constraints of minimum temperature difference for heat exchange. There exists an optimum combination of compressor inlet pressure, compressor outlet pressure and split ratio for supercritical CO₂ Brayton cycles, in which case, the cycle efficiency reaches the maximum.