The mathematic model for tower type solar thermal power generation system with indirect-heated supercritical CO_2 Brayton cycles was established. The system is simulated under conditions with different turbine inlet temperatures

main compressor outlet pressures and pressure ratios

and the variation law of thermal efficiency of the whole plant was analyzed. The results show that

along with the rising of the turbine inlet temperature

the thermal efficiency of the whole plant first increases and then decreases

and it would reach the maximum in the vicinity of 750 ℃. With an increase in the circulating pressure ratio

the thermal efficiency of the whole plant increases firstly and then decreases. There exists the optimal pressure ratio for different outlet pressures of the main compressor and different outlet temperatures of the turbine. Furthermore

two optimization proposals of the system were proposed

and the optimized parameter values are given: the main compressor inlet pressure of 20～35 MPa

and the turbine inlet temperature of 500～850 ℃.