Abstract: The main cryo-condensation pump of J type is employed by neutral beam injector (NBI). The radiation shield of the pump is designed based on the influence of its structure on the radiation heat transfer and the pumping speed. By tracking each packet of the thermal radiation, a Monte Carlo method was adopted to calculate the radiation heat transfer, and to obtain the relationship between the radiation heat transfer and the structure using the control variate method. The simulation results show that the emissivity of the walls and the height of structural units have a great influence on the radiation heat transfer. However, the boundary shape of the structural unit has little effects on the radiation heat transfer. The results will provide reference for the design of the cryo-condensation pump in EAST-NBI and the similar built-in cryo-condensation pump of J type.