Abstract: Previous studies have shown that the let-7a was an abundance miRNA in the spleen of common carp, but its function in the carp spleen is still not clear. Therefore, in this study, the characteristic of let-7a mature sequence were analyzed, the expression profiles of let-7a were determined by RT-qPCR in eight tissue samples at two stages of deve-lopment, and the bioinformatic analysis were performed for the predicted target gene of let-7a. The results showed that the let-7a existed sequence length variation and base substitution in the spleen of common carp. The let-7a length ranged from 18 nt to 25 nt, and the base substitution types of its mature sequence had nine. Especially, the base substitution rate in seed sequence was 2.1%. The expression profile analysis showed that the let-7a as a generalized high abundance miRNA exhibited striking temporal expression characteristics. The let-7a expression levels in the immunity related organs including gill, intestine, spleen and kidney were significantly higher than those in the other organs. GO enrichment analysis suggested that the let-7a significantly involved in the regulation of transcription, cellular response to stimulus, ion transport, transmembrane transport, protein amino acid phosphorylation, cell motion, and so on. Especially, the predicted target genes of let-7a were significantly enriched in many biological processes of stimulus response. In a word, let-7a can widely participate in the regulation of various biological processes, especially in the stimulus response of the body in common carp.