The main objective of the study will be to assess the efficacy and safety of Allisartan Isoproxil/sustained-release indapamide (240 mg/1.5 mg) in patients with mild to moderate essential hypertension uncontrolled after 4-week treatment with Allisartan Isoproxil (240 mg).

Allisartan isoproxil is a novel angiotensin receptor blocke available in China for over ten years and has the same active compound EXP3174 with losartan, while sustained-release indapamide is a long-standing thiazide-type diuretic. The novel single-pill combination of allisartan isoproxil and sustained-release indapamide exerts synergistic antihypertensive effects.

Key Inclusion Criteria:

1. Patients 18-75 years old with mild to moderate essential hypertension;

2. Patients who meet one of the following criteria when screening:

   1. Untreated patients (either newly diagnosed essential hypertension or those patients with a history of hypertension but have not been taking any antihypertensive drugs for at least 2 weeks before the first visit) must have an office msSBP ≥ 150 mmHg and < 180 mmHg and msDBP<110 mmHg;
   2. Patients who had not received regular treatment with allisartan isoproxil (240 mg/day) (less than 4 weeks of medication intake or a history of missing doses for 5 or more days within the 4 weeks before screening) with the msSBP of 140 mmHg≤ SBP <180 mmHg and DBP<110 mmHg;
   3. Patients currently receiving other antihypertensive agents (non-study medications for at least 2 weeks prior to screening) with the msSBP of 140 mmHg≤ SBP <180 mmHg and DBP<110 mmHg, and the clinician determined that it was appropriate to switch to allisartan isoproxil (240 mg/day);
   4. Patients who have been stably treated with allisartan isoproxil (240 mg/day) for at least 4 weeks with the msSBP of 140 mmHg≤ SBP <180 mmHg and DBP<110 mmHg.

3. During randomization for the double-blind treatment period, msSBP should be 140 mmHg≤ SBP <180 mmHg and DBP<110 mmHg;

4. Participants enrolled in the ABPM study should have 24-hour mean ambulatory blood pressure ≥130/80 mmHg after 4 weeks of monotherapy treatment;

5. Patients who understand and sign the informed consent form.

Key Exclusion Criteria:

1. Patients with secondary hypertension;
2. Patients with msSBP ≥180 m mHg and/or msDBP≥110 mmHg, or withhypertensive emergency or hypertensive urgency.
3. Patients who have taken three or more antihypertensive drugs (including single-pill combination) simultaneously within one month prior to screening.
4. Patients with history of heart failure (New York Heart Association (NYHA) Functional Classification class III and IV), acute coronary syndrome, percutaneous coronary intervention, or other serious heart diseases (such as cardiogenic shock, moderate or severe heart valvular disorders, atrioventricular block second or third degree, bradycardia (with a heart rate <50 beats per minute), severe arrhythmias) in the past 6 months.
5. Patients with history of severe cerebrovascular diseases (such as hypertensive encephalopathy, cerebrovascular injury, stroke, transient ischemic attack) in the past 6 months.
6. Patients with aortic aneurysm, aortic dissection, or dissecting aneurysm.
7. Patients with severe renal insufficiency (Cr>1.5 times the upper limit of normal).
8. Patients with hypokalemia or hyperkalemia.