Using FLUENT software to numerically simulate the gas turbine casing, the airflow distribution inside the casing and the defects of high wall temperature on the casing are obtained. Therefore, on this basis, the cooling structure of the gas turbine is optimized from two aspects. The branch structure is set up inside the box body to improve the airflow direction, and the exhaust ejector device is set up outside to cool the box body and reduce the energy consumption. After that, the ejection efficiency is increased by slotting the trailing edge of the nozzle to solve the problem of high temperature inside the box body. The results show that the branch structure can improve the internal flow field distribution of the box, and the cooling effect of the box is very obvious. When the flow air flow at the nozzle of the exhaust ejector is used as the power source, the ejection efficiency is 31.9 %, which plays a certain role in the cooling of the box, but it can not meet the cooling requirements of the bottom plate ; when the nozzle is slotted at the trailing edge ( 6 slots ), the ejection efficiency can reach 41.2 %, which meets the cooling requirements of the bottom plate.