Hypoplastic left heart syndrome (HLHS) is a severe congenital heart defect characterized by underdevelopment of the left ventricle (LV), mitral valve, aortic valve, and ascending aorta. Based on the "no flow, no grow" theory, this protocol describes a reproducible fetal sheep model that restricts mitral inflow to impair LV growth in utero. At approximately 120 days of gestation, maternal laparotomy and hysterotomy are performed, followed by fetal left thoracotomy. A balloon catheter is introduced into the left atrium (LA), left deflated at surgery, and subsequently inflated beginning at approximately postoperative day 3 to restrict mitral inflow until antegrade flow in the ascending aorta is markedly reduced or abolished, as verified by a flow probe after maternal recovery. Hemodynamic changes are continuously monitored via fetal carotid arterial pressure and an ascending aortic flow probe. Fetuses are maintained for up to three weeks and undergo necropsy. Critical steps include atraumatic placement and securement of the aortic flow probe and the LA balloon in the fragile fetus, as well as meticulous prevention of amniotic fluid loss before and after uterine closure. This model provides a platform for investigating HLHS mechanisms, validating imaging biomarkers, evaluating prenatal or perinatal therapeutic interventions, and guiding the development of preventive strategies.