In mammal, CYP1A has attracted special attention due to its important roles in the oxidative metabolism. In fish, the researches on CYP1A are more focus on its roles in pollution in water environments, but the immune function is unclear. In the study, CiCYP1A gene was cloned from grass carp (Ctenopharyngodon idella). Tissue distribution exhibited an overwhelmingly high basal expression levels in the liver. After GCRV infection, CiCYP1A showed a potent response, indicating CiCYP1A was involved in GCRV-induced immunity. Subcellular localisation showed CiCYP1A was distributed in the cytoplasm. Besides, dual-luciferase activity assays revealed CYP1A was relevant for IFN-I signaling pathway modulation, furthermore, overexpressed CYP1A potently suppressed the mRNA expression of IRF3 and IFN-I but not IRF7. The results provide new sights into exploring immune function of CiCYP1A in teleosts.