For an inertial-confinement-fusion cryogenic target, the fusion ice layer inside the capsule should have a uniformity more than 99% and an inner surface roughness less than 1 μm (root mean square) to avoid Rayleigh-Taylor instabilities. And this highly smooth ice layer required for ignition is generated at the presence of volumetric heat and affected by the thermal environment around the capsule. For the D₂ fuel targets, the volumetrically heating can be supplied by exposing the ice layer to IR radiation. A major challenge of IR-layering is a spherically uniform IR illumination of the capsule, particularly for capsules held in cylindrical hohlraums in indirect-drive targets. In the present study, a numerical study is conducted on the thermal environment of D₂ fuel capsule under directional infrared radiation. A 3D simulation model coupling the photonic and thermal fields has been established based on the Monte Carlo ray tracing method. The influence of infrared optical fibers’ layouts and source power intensity on temperature characteristics outside the capsule have been studied. The results indicate that at constant total power of optical fibers, the more optical fibers set up, the lower the average temperature of the outer surface decreases, and the more uniform the capsule outer surface temperature turns to be. The temperature uniformity of capsule outer surface deteriorates with the two or less optical fibers, but improves with the number greater than 2 in which case ΔT_max and ΔT_w can decrease by 61.94% and 76.33% at most. A proper offset of optical fiber spots towards the hohlraum poles can improve the temperature uniformity of capsule outer surface, while the other two optical fiber spot offset schemes deteriorate the temperature uniformity. The results are of guiding significance for determining the optical fibers layout in experiment and further design option for cryogenic targets.