The preproinsulin gene encodes a precursor protein of insulin, which is the most important hormone for lowering blood glucose levels and promoting the synthesis of glycogen, fat and protein. To explore the correlation between polymorphisms in the preproinsulin gene and growth traits in grass carp, the preproinsulin gene sequence, measuring a total of 5708 bp, was identified in the grass carp genome.The sequence includes a promoter, two introns and three exons, and encodes a 108-aa protein. A total of three SNPs were identified, including SNP1(g.-2661C > G) in the promoter and SNP2 (g.1305G > C) and SNP3 (g.1682G > A) in intron 2. The correlation between SNPs and growth traits in grass carp was analysed by a general linear model (GLM). The results indicated that no genotype in each single SNP, SNP1 with SNP2, or SNP1 with SNP3 was related to rapid growth and low fatness, respectively. While eight genotypes of SNP1, SNP2 and SNP3 were combined into six types of effective diplotypes, the H5 diplotype was significantly superior to the other diplotypes (P < 0.05) concerning body weight, body length, body height and body width, and its fatness was lower than those of the other diplotypes, except for H6 diplotype. This result indicated that the H5 diplotype of the preproinsulin gene in grass carp may be a candidate molecular marker for selecting fast-growing and low-fatness grass carp.