We aimed to investigate the relationship of time-averaged on-treatment systolic blood pressure (SBP) with the risk of first stroke in the CSPPT (China Stroke Primary Prevention Trial). A post hoc analysis was conducted using data from 17720 hypertensive adults without cardiovascular disease, diabetes mellitus, and renal function decline from the CSPPT, a randomized double-blind controlled trial. The primary outcome was first stroke. Over a median follow-up duration of 4.5 years, the association between averaged on-treatment SBP and risk for first stoke followed a U-shape curve, with increased risk above and below the reference range of 120 to 130 mmHg. Compared with participants with time-averaged on-treatment SBP at 120 to 130 mmHg (mean, 126.2 mmHg), the risk of first stroke was not only increased in participants with SBP at 130 to 135 mmHg (mean, 132.6 mmHg; 1.5% versus 0.8%; hazard ratio, 1.63; 95% confidence interval, 1.01-2.63) or 135 to 140 mmHg (mean, 137.5 mmHg; 1.9% versus 0.8%; hazard ratio, 1.85; 95% confidence interval, 1.17-2.93), but also increased in participants with SBP <120 mmHg (mean, 116.7 mmHg; 3.1% versus 0.8%; hazard ratio, 4.37; 95% confidence interval, 2.10-9.07). Similar results were found in various subgroups stratified by age, sex, and treatment group. Furthermore, lower diastolic blood pressure was associated with lower risk of stroke, with a plateau at a time-average on-treatment diastolic blood pressure <80 mmHg. In conclusion, among adults with hypertension and without a history of stroke or myocardial infarction, diabetes mellitus, or renal function decline, a lower SBP goal of 120 to 130 mmHg, as compared with a target SBP of 130 to 140 mmHg or <120 mmHg, resulted in the lowest risk of first stroke.