The hotspot temperature rise of reactor is one of the important indexes in reactor design. The proper hotspot value can ensure that the reactor will not overheat failure and threaten the safety of power system. Therefore, the hotspot temperature rise is taken as the optimization goal. Firstly, MATLAB and COMSOL co-simulation methods are used to complete the reactor modeling and the simulation solution of temperature field, and the hotspot temperature of the reactor is obtained. Then the response surface function of the hotspot temperature and its related variables is constructed by the response surface method, and the rationality and accuracy of the response surface function are analyzed by simplifying the response surface model according to variance analysis. Finally, the optimal solution of the function is obtained by using the mathematical programming method. The results show that the hotspot temperature of the reactor envelope winding decreases obviously after optimization.