The cyclic nucleotide-gated ion channel (CNGC), as a non-selective cation channel, plays a pivotal role in plant growth and stress response. A systematic analysis and identification of the BnCNGC gene family in Brassica napus is crucial for uncovering its biological functions and potential applications in plant science. In this study, we identified 61 BnCNGC members in the B. napus genome, which are phylogenetically similar to Arabidopsis and can be classified into Groups I-IV (with Group IV further subdivided into IV-a and IV-b). Collinearity analysis with other species provided insights into the evolution of BnCNGC. By homology modeling, we predicted the three-dimensional structure of BnCNGC proteins and analyzed cis-acting elements in their promoters, revealing diverse roles in hormone regulation, growth, and stress response. Notably, overexpression of BnCNGC57 (BnaC09g42460D) significantly increased seed size, possibly through regulating cell proliferation via the MAPK signaling pathway. Our findings contribute to a better understanding of the BnCNGC gene family and highlight the potential regulatory role of BnCNGC57 in the seed development of B. napus.