Abstract: To further study the molecular characteristics and functions of complement activation regulators in teleost fish, we cloned the CD46 gene of rainbow trout (Oncorhynchus mykiss) and analyzed its molecular characteristics systematically. The results showed that the CD46 gene of rainbow trout was composed of 10 exons and 9 introns; the cDNA sequence of CD46 was 2812 bp and encoded 317 amino acids; the protein sequence of CD46 was composed of 1 signal peptide, 4 SCR domains, 1 transmembrane region and 1 intracellular region; and the predicted molecular weight of CD46 was 33.9 kD. Synteny analyses showed that the rainbow trout CD46 gene was located on chromosome 16, and its gene locus had conserved synteny in vertebrates. The expression analyses showed that the rainbow trout CD46 gene was expressed in various tissues and leukocyte subpopulations. To further clarify the immune function of rainbow trout CD46, the GST protein and the fusion protein GST-CD46 were prokaryotically expressed and purified. The experiment of hemolytic activity showed that, compared with GST, GST-CD46 could significantly inhibit the hemolytic activity of rainbow trout serum to rabbit erythrocytes in a dose-dependent manner, indicating that rainbow trout CD46 is a regulator of complement activation. In addition, GFP or GFP-CD46 were overexpressed in HEK293T cells. The experiment of cell damage showed that, compared with GFP, GFP-CD46 could significantly inhibit the damage of rainbow trout serum to HEK293T cells, further indicating that rainbow trout CD46 is a regulator of complement activation and can protect cells from the damage of complement system. In conclusion, this study not only increased the knowledge of the molecular characteristics and regulatory function of rainbow trout CD46, but also provided a theoretical basis for the in-depth study of the immune function of this molecule and its application in disease resistance.