Congenital atrioventricular septal defects are a complex group of cardiac malformations. Previously, this malformation was referred to as atrioventricular canal malformation or common atrioventricular channel. The condition is a fetal abnormality in the development of the endocardial cushions, resulting in a defect in the primary foramen above the atrioventricular valve or a perimembranous septal defect below the atrioventricular valve and varying degrees of splitting of the atrioventricular valve and annulus. Depending on the degree of endocardial cushion defect, the condition is clinically classified as simple or partial atrioventricular septal defect and complete atrioventricular septal defect. This article focuses on partial AV septal defects. The pathophysiology and clinical changes of partial AV septal defects depend on the size of the primary orifice defect and the degree of mitral regurgitation. In simple primary orifice defects, the primary orifice is not large, there is no mitral regurgitation, only a left-to-right atrial horizontal shunt or a mild mitral mega-valve fracture, and mitral regurgitation is not obvious, and the symptoms are similar to those of atrial septal defects. When the primary orifice defect is large and mitral regurgitation is obvious, the left-to-right shunt flow is large, and the clinical manifestations are recurrent respiratory infections, pneumonia, heart failure, shortness of breath and excessive sweating, backward growth and development, low activity, and easy development of large right and left hearts, progressive pulmonary hypertension, and obstructive lesions of small pulmonary vessels. Diagnosis is based on clinical symptoms and signs, but chest X-ray, electrocardiogram, and Doppler color ultrasound are needed to clarify the diagnosis and the extent of lesions, and cardiac catheterization and cardiovascular angiography are needed for those suspected of having moderate or higher pulmonary hypertension. Patients with simple primary foramen ovale defect, if the atrial defect is not large and without pulmonary hypertension can be operated before school age; patients with partial-type atrial septal defect with large mitral valve cleft, most of whom have varying degrees of mitral regurgitation, should all be operated early. Surgery is performed under moderate hypothermia (28°C anal temperature) with extracorporeal cardiac arrest via a right atrial incision for repair of the primary foramen ovale defect patch and suturing of the mitral valve mega-valvular fissure. The surgical results are similar to those of the secondary type of atrial septal defect in cases of pure primary orifice atrial septal defect. The surgical outcome of partial atrial septal defects is importantly related to the anatomy and function of the mitral valve. The operative mortality rate for partial atrioventricular septal defect without left atrioventricular regurgitation has been reported to be only 0.4%; with moderate to severe atrioventricular regurgitation, the operative mortality rate can be as high as 4%. The author recently performed several cases of partial atrioventricular septal defect repair with good surgical results, and all patients were discharged from the hospital.