Background Preeclampsia is a pregnancy complication characterized by high blood pressure and signs of damage to another organ system, often the kidneys. Recent studies suggest that endothelial dysfunction and trophoblast apoptosis are involved in the pathogenesis of preeclampsia. Purpose To investigate the effects of tanshinone IIA (TIIA) on clinical symptoms, vascular endothelial function, and placental trophoblast apoptosis in preeclampsia rats. Materials and Methods Twenty-four pregnant Sprague-Dawley rats, between 8 and 10 weeks old, were randomly divided and allocated into three groups: Control, model, and treatment, with each group consisting of 8 rats. The control group received normal saline injections via the tail vein; the model group received NG-nitro-l-arginine methyl ester (l-NAME) to induce preeclampsia, followed by normal saline injections; and the treatment group received TIIA (10 mg/kg) via tail vein injection after preeclampsia induction with l-NAME. Various parameters were measured, including tail artery systolic pressure, 24-hour proteinuria, offspring and placental weight, levels of endothelial nitric oxide synthase (eNOS), endothelin-1 (ET-1), placental growth factor (PLGF), and soluble FMS-like tyrosine kinase 1 (sFlt-1) in serum, and expression of B-cell lymphoma-2 (Bcl-2) and Bcl-2-related X protein (Bax) in placental trophoblasts. Results TIIA treatment led to decreased tail artery systolic pressure and 24-hour urine protein levels, increased body and placental weights of offspring, and favorable changes in serum levels of ET-1, sFlt-1, eNOS, and PLGF. Moreover, TIIA treatment resulted in decreased Bax levels and apoptosis in placental trophoblasts, along with increased Bcl-2 levels. Conclusion TIIA can improve the clinical symptoms of preeclampsia in rats induced by l-NAME, alleviate the apoptosis of placental trophoblast cells, and improve vascular endothelial function.