Objective To investigate the changing tendency of mitral valve coaptation area and coaptation index of moderate mitral regurgitation （MR） in a dog experiment，and provide evidence for predicting long-term surgical results. Methods Real-time three-dimensional transesophogeal echocardiography （RT-3D-TEE） images were obtained in 15 dogs via Philips IE33 echocardiography system，and animal experiment model was established. RT-3D-TEE images were taken by gradually narrowing the ascending aorta and increasing left ventricular pressure till moderate MR. Original data were analyzed using Philips Qlab 7.0 three-dimensional quantification software，and mitral valve coaptation area and coaptation index were calculated. Specimen coaptation index of the mitral leaflets was calculated after the animal experiment. Cutoff values of coaptation index and left ventricular pressure were calculated by receiver operating characteristic （ROC） curve. Results There was statistical difference in coaptation area （198±50）mm2 vs. （123±36）mm2，P＜0.05） and coaptationindex （0.25±0.06 vs. 0.13±0.03，P＜0.05） between non-MR state and MR status of the 15 dogs. The area under the ROC curve of coaptation index and moderate MR was 0.879±0.019 with 95% CI 0.843 to 0.916，and the cutoff value was 0.213（P＜0.05）. The area under the ROC curve of left ventricular pressure and moderate MR was 0.882±0.021 swith 95% CI 0.840 to 0.923，and the cutoff value was 225 （P＜0.05）. There was no statistical difference between specimen mitral valve area and early-diastolic mitral leaflet area，specimen coaptation area and coaptation area，specimen coaptation index and coaptation index （P＞0.05）. Early-diastolic mitral leaflet area was significantly correlated with specimen mitral valve area （r=0.937，P＜0.05）. Coaptation area was significantly correlated with specimen coaptation area （r=0.917，P＜0.05）. Coaptation index was significantly correlated with specimen coaptation index （r=0.946，P＜0.05）. The correlation of coaptation index and specimen coaptation index was higher than those of coaptation area and specimen coaptation area，and earlydiastolic mitral leaflet area and specimen mitral valve area. Conclusions Both coaptation area and coaptation index significantly decrease in MR status. Coaptation index can more precisely reflect MR degree，and provide reference for prognosis of mitral valve repair. RT-3D TEE can accurately measure mitral valve coaptation area and coaptation index.