[en] The effect of helium on the irradiation damage characteristics and radiation induced segregation at grain boundaries in the welding heat affected zone (HAZ) of Fe-Cr-Mn(W, V) alloy for fusion-fission mixed reactor was investigated by super-high voltage electron microscopy connected with a high energy ion accelerator; Experimental results show that the existence of helium enhanced the density of dislocation loop during initial irradiation stage and promoted formation of void nuclei and increase of void swelling, effectively suppressed concentration decrease of solute atoms Cr, Mn at grain boundaries meanwhile the concentration change of Ni, V, Si, W at grain boundary is also suppressed. The HAZ of Fe-Cr-Mn (W, V) alloy has superior resistance to irradiation damage under helium atmosphere