Objective Real time three-dimensional echocardiography (RT-3DE) was used to observe the three-dimensional shape of mitral valve elongation and the hemodynamic changes of the left ventricular outflow tract, and to analyze the causes of left ventricular outflow tract dynamic obstruction caused by anterior mitral valve elongation. Methods From February 2018 to March 2020, 18 patients with mitral valve elongation (Study Group) and 23 normal people (control group) were diagnosed by echocardiography in our hospital. On the basis of conventional two-dimensional echocardiography, RT-3DE was used to observe the three-dimensional shape and hemodynamic changes of malformed mitral valve. The length of anterior mitral valve (AMV-L), left ventricular outflow velocity (LVOT-V), pressure gradient (LVOT-P), interventricular septal thickness (IVS-D), left ventricular posterior wall thickness (LVPW-D), left ventricular end diastolic diameter (LVD-D), left ventricular end systolic diameter (LVS-D), mitral velocity (MV), left ventricular ejection fraction (LVEF) and heart rate (HR) were measured in the two groups. The Length of residual part of anterior mitral valve elongation (AMV-eL) was measured in the study group. Results ①The AMV-L, LVOT-V, LVOT-P, IVS-D and LVPW-D in the study group were higher than those in the control group (P < 0.05), but there was no significant difference in LVD-D, LVS-D, MV, LVEF and HR between the two groups (P > 0.05). ②The systolic anterior motion (SAM) of anterior mitral valve elongation was found in the study group. ③There was a positive correlation between AMV-L and AMV-eL in the study group (r=0.925,P＜0.05), and between AMV-eL and LVOT-V, LVOT-P (r1=0.916,r2=0.923,P＜0.05). Conclusion Mitral valve elongation can lead to left ventricular outflow tract obstruction. The SAM of anterior mitral valve elongation is the cause of left ventricular outflow tract obstruction. The degree of obstruction depends on the AMV-L and AMV-eL.