ABSTRACT Grass carp ( Ctenopharyngodon idellus ) is a vital species in China's aquaculture industry but is highly susceptible to grass carp haemorrhagic disease (GCHD) caused by the grass carp reovirus (GCRV). GCRV infection in Ctenopharyngodon idellus kidney cells (CIK) induces reactive oxygen species (ROS) overproduction, triggering oxidative stress. Catalase (Cat) is a crucial antioxidant enzyme that decomposes hydrogen peroxide (H 2 O 2 ) into H 2 O and O 2 . In this study, we investigated and characterised the roles of CiCat in grass carp. CiCat is expressed ubiquitously in all examined tissues. Furthermore, when challenged with GCRV, its expression levels are significantly upregulated. Fluorescence analysis revealed that CiCat exhibits a widespread cellular distribution, while GCRV infection enhances its aggregation and co‐localisation with mitochondria (Mit) and endoplasmic reticulum (ER). Overexpression of CiCat eliminates both H 2 O 2 and GCRV‐induced ROS, decreases the expression of antioxidant enzymes and promotes GCRV replication. Further research showed that overexpression of interferon regulatory factor‐7 (IRF‐7) inhibits GCRV replication, but CiCat reduces IRF‐7 expression. In summary, CiCat promotes GCRV replication by reducing CiIRF‐7 expression.