Objective To investigate the role of the endoplasmic reticulum (ER) stress-mitochondrial autophagy pathway in pulmonary arterial hypertension ( PAH) in rats by inhibiting ER stress induced by monocrotaline. Methods SD rats were randomly divided into three groups (15 rats per group): normal control group, group with PAH induced by monocrotaline, and 4-phenylbutyric acid (4-PBA) group. The mean pulmonary artery pressure (mPAP) and mean right ventricular pressure (mRVP) in the rats were measured by the Biological Information Acquisition System. The remodeling of pulmonary vessels was observed by hematoxylin-eosin (HE) staining, while ImageJ was used to measure indices related to pulmonary vascular remodeling. The ultrastructure of mitochondria in pulmonary artery smooth muscle cells was observed by electron microscopy. Quantitative real-time PCR analysis was used to measure the mRNA levels of the key factors in the ER stress-mitochondrial autophagy pathway. Results (1) mPAP and mRVP increased in the PAH group (P<0.001). However, after 4-PBA intervention, mPAP and mRVP decreased compared with those in the PAH group (P<0.001). (2) HE staining showed clear remodeling of pulmonary small vessels in the PAH group, and the measurement of vascular remodeling indices indicated increased thickness of pulmonary arteriole media and lumen stenosis (P< 0.05). ( 3) The mitochondria of pulmonary artery smooth muscle cells in the PAH group were swollen, the mitochondrial crista structure had dissolved or disappeared, and mitochondrial autophagy was increased, as revealed by electron microscopy. After the ES stress was inhibited, destruction of the mitochondrial structure and mitochondrial autophagy were reduced. (4) Compared with those in the NC group, the mRNA expression levels of ERS-related factors PERK, ATF4, Bcl-2 and CHOP were increased in the PAH group (P<0.001), mitochondrial fusion factor Mfn2 decreased (P<0.001), mitochondrial fission factor Drp1 increased (P<0.001), and the expression of Atg12, LC3 and p62 increased in the PAH group (P<0.001). After rats had been administered 4-PBA, the mRNA expression levels of ERS-related factors decreased (P<0.001), the mRNA level of Mfn2 increased (P<0.001), while that of Drp1 decreased (P<0.001). Moreover, the mRNA expression levels of mitochondrial autophagy factors Atg12, LC3 and P62 (P<0.01) decreased. Conclusions ERS may promote the establishment and progression of pulmonary hypertension through the mitochondrial autophagy pathway. This may provide a new concept for the treatment and prevention of pulmonary hypertension.