Reduced activation ferritic/martensitic (RAFM) steel, as a typical body centered cubic (bcc) iron based structure material, has become a candidate material for future fusion reactor. Nano-scale prismatic interstitial dislocation loops formed in irradiated RAFM have been studied for many years because of their significant influences on the mechanical properties (e.g. irradiation embrittlement, hardening, creep, etc.). Compared with edge dislocation, screw dislocation has very important influence on plastic deformation behavior because of its low mobility. Thus, the mechanism of interaction between screw dislocation and interstitial dislocation loops has become an intense research topic of interest. In this study, the slip behavior of screw dislocation and the mechanisms of interaction between screw dislocation and ½[ $11\bar 1$ ] dislocation loop in bcc-Fe at different temperatures are investigated by molecular dynamics simulation. The results show that the screw dislocation mainly slides along the ( $\bar 2 11$ ) plane at a low temperature of 2 K under the increase of shear stress. With the temperature increasing to 823 K, it is prone to cross slip, and then the cross slip occurs alternately in the ( $\bar 1 10$ ) plane and the ( $\bar 2 11$ ) plane. Therefore, with the increase of temperature, the critical shear stress decreases gradually. When the screw dislocation slips close to the dislocation loop, the mechanism of interaction between screw dislocation and dislocation loop is different at different temperature: at low temperature of 2 K, there is repulsive force between screw dislocation and dislocation loop, when screw dislocation slip approaches to the dislocation loop, the cross slip of screw dislocation can occur, and shear stress is lower than that from the model without dislocation loop; at medium temperatures of 300 K and 600 K, the influence of repulsive force on the cross slip of screw dislocation can be weakened, and screw dislocation will slip through the dislocation loop then form the new structure named helix turn, which further hinders screw dislocation slipping and results in the increase of shear stress; at a high temperature of 823 K, the screw dislocation is more likely to cross slip due to the thermal activation, and the slip of dislocation loop is also easier to occur, but the screw dislocation and the dislocation loop do not contact each other in the whole shearing process, therefore the shear stress is lowest.