A novel missense variant (NM_005327.7: c.99C>G, p.Ile33Met) was discovered in 3-hydroxyacyl-CoA dehydrogenase (HADH), which is involved in congenital hyperinsulinism (CHI). This variant may be damaging or deleterious, as assessed using protein prediction software. This study aimed at the impact of this variant on islets and if it caused the leu-sensitive insulin secretion. The adenoassociated virus containing the HADH missense variant (p.Ile33Met), wild-type (WT) HADH or empty vector (EV) was constructed, and the rats were infected with it. Three weeks after the transfection, 15 rats were dissected to observe the effect of the variant on the islet tissue. Then we treated the remaining rats with leucine or sodium carboxymethyl cellulose (CMC-Na) by gavage and drew blood from the rat tail vein to detect the variations in blood glucose, serum insulin and serum glucagon. Further, we dissected the rats to observe the fluctuation of insulin and glucagon contents in pancreatic islets under the combined action of leucine and p.Ile33Met. Insulin and glucagon were observed in the islet tissue under an inverted fluorescence microscope, serum insulin and glucagon were detected by ELISA, and the blood glucose value was determined using a Roche glucometer. The positive area and average gray value of islet fluorescence pictures were analysed using the software Image J (USA). Rats expressing p.Ile33Met showed significantly higher insulin and glucagon content, as well as the islet area, compared to WT and EV rats. Moreover, after intragastric administration of leucine, the serum insulin content of the variant rats increased but the blood sugar level decreased significantly. Meanwhile, there was an appreciable decrease in the insulin content in rat pancreatic islet tissues. Our results suggest that the variant NM_005327.7: c.99C>G promotes the proliferation of pancreatic islets, enhances the secretion of insulin, and induces leu-sensitive hyperinsulinaemia.