A certain type of IR FPA device needs very short cooling start-up time in engineering application,and the time is related to the cooling performance of J-T coolers and the thermodynamics performance of Dewars.In the paper,without changing the condition of the J-T cooler,through the optimization and simulation of thermal load design of Dewar structure parts such as cold shield,frame,cold table,et al,the thermal quality of the cooling head is reduced,the heat conduction efficiency is improved,and the cooling start-up time of the device is effectively reduced .