Inclusion Criteria:  Subject must be age ≥18 years; Subject has mPAP 21-24 mmHg, and PAWP<15mmHg.The underlying diseases that cause critical PAH belong to the first group, which is divided into: Idiopathic pulmonary hypertension, hereditary pulmonary hypertension, drugs and poisons associated with pulmonary hypertension, connective tissue diseases associated with pulmonary hypertension, HIV infection associated with pulmonary hypertension, portal hypertension associated with pulmonary hypertension, tumors associated with pulmonary hypertension, congenital heart disease associated with pulmonary hypertension. Subject (or legal guardian) understands the trial design and treatment procedures and provides written informal consent before any trial-specific tests or procedures are performed.  Exclusion Criteria:  Pulmonary hypertension (PH) confirmed by right heart catheter (RHC) before enrolment, i.e. mPAP ≥25 mmHg at rest. Ongoing or a history of >2 weeks of continued use of therapies that are considered definitive PH treatment: endothelin receptor antagonists (ERA; e.g. bosentan, ambrisentan), phosphodiesterase type 5 inhibitors (PDE5; e.g. sildenafil, tadalafil, vardenafil), prostanoids (e.g. epoprostenol, treprostinil, iloprost, beraprost) and soluble guanylate cyclase stimulator (e.g. Riociguat). Intermittent use of PDE5 inhibitors for male erectile dysfunction is permitted. Known intolerance to ambrisentan or one of its excipients. Pulmonary vein occlusive disease Pulmonary capillary hemangiomatosis Surgical repair or interventional occlusion of congenital heart disease within 6 months prior to screening of this study Active connective tissue diseases Pulmonary hypertension due to left heart disease Pulmonary hypertension due to pulmonary disease and/or hypoxia Acute pulmonary embolism and/or chronic thromboembolism Clinically significant anemia, defined as hemoglobin concentration 75% below the normal lower limit. Renal insufficiency was defined as glomerular filtration rate [EGFR] <30 mL/min/1.73m2. Transaminase (ALT and/or AST) increased, exceeding the upper limit of normal value by 3 times. Arterial systolic blood pressure < 85 mmHg. Uncontrolled hypertension, defined as blood pressure >160/90 mmHg (resting state) and/or >220/120 mmHg (load state). Participate in any drug clinical trial within 4 weeks prior to screening in this study and/or plan to participate in another drug clinical trial during the study period. Pregnant or lactating women.