The present invention relates to a composition comprising a protein phosphatase 1 inhibitory peptide for treating vascular diseases. The composition of the present invention inhibits protein phosphatase 1 (PP1)-mediated dephosphorylation to suppress abnormal proliferation of vascular smooth muscle cells (VSMCs), and activates eNOS of vascular endothelial cells (VECs) to induce the recovery from dysfunction, and thus can be favorably used in the treatment of vascular diseases including pulmonary hypertension.