Solar-coal complementary electric generation is a new way to utilize solar thermal, however, using solar thermal to replace the heat transfer of some heating surfaces will inevitably affects the stability and efficiency of boiler operation, and the running characteristics of the boiler will affects the utilization capacity of the solar thermal in turn. A 300MW unit thermal- circulation boiler has been studied. A research on calculation of heat transfer characteristic has been made. The influence of solar-energy utilization on heat transfer has been analyzed. Calculation and analysis indicate that using solar thermal to heat the feedwater can decrease coal consumption. 87.2% of the coal consumption decrement can be attributed to the replacement of coal by solar thermal, 12.8% of the coal consumption decrement is leaded by the boiler efficiency increment; Solar thermal should not be introduced into the boilers with low exhaust gas temperature and the boilers running at low load operation; Using solar thermal to heat the feedwater leads to decreasing spray water; Through the boiler calculation in this paper, the maximum solar thermal utilization capacity is 19.27MW.