Due to the large size and low operating temperature of the ultra-large diameter in-orbit assembly infrared telescope, mutual interference between assembly modules, the traditional thermal control methods cannot fully meet its requirements. In order to meet the proposed thermal design index, the external thermal environment analysis of the SE-L2 orbit was completed, and a five-layer sunshield for the ultra-large diameter in-orbit assembly infrared telescope was designed. The telescope finite element model was established and then simulated with the UG software. The simulation results show that after being shaded by the sunshield, the intensity of the thermal radiation from the sun, which is 1 296 W/m2, reduces to 0.036 W/m2 when it reaches the low temperature area. 210 days after the sunshield is unfolded, the temperature of this kind of telescope reduces to less than 50 K, through passive cooling radiation, meeting the demand of thermal control. This design is a valuable reference on the Chinese future construction of ultra-large space telescope because of the research on its thermal control.