Although short-term exposure to fine particulate matter (PM_2.5) air pollution has been shown to induce elevated blood pressure (BP), limited evidence is available regarding the association between ambient PM_2.5 and BP levels in nationwide China and how the association may change. This study sought to explore acute BP changes with exposure to PM_2.5 at levels below China’s current National Ambient Air Quality Standards (NAAQS). Based on a spatiotemporal study of over 1 million adults, we linked BP measurements to daily estimates of PM_2.5 from multiple sources (i.e. in situ observations, satellite measurements of aerosol and numeric simulations of air quality model) after adjusting for several individual-level covariates and further conduced the below-criteria models by restricting the analyses within subsets of individuals with short-term PM_2.5 exposure below 75 μg m⁻³ (i.e. NAAQS of 24 h PM_2.5 in China). We further explored variations in BP-PM_2.5 associations by pollution level and for different demographic groups. With full adjustments, a 10 μg m⁻³ increase in PM_2.5 was statistically significantly associated with a 0.049 mmHg (95% confidence interval, CI: 0.041, 0.057) increase in systolic BP, a 0.022 mmHg (95% CI: 0.017, 0.027) increase in diastolic BP and a 0.77% (95% CI: 0.62%, 0.92%) increased risk of hypertension (HPN). For both BP and HPN, the exposure-response curves were linear, with no threshold effects, at the low-concentration ends and sublinear at the high-concentration ends. Below the current NAAQS, the associations in population level remained statistically significant and were even stronger. A 10 μg m⁻³ increase in below-NAAQS PM_2.5 was associated with a 1.95% (95% CI: 1.44%, 2.47%) increased risk of HPN. Specific subpopulations were more susceptible to PM_2.5 exposure. These findings can help support decisions by policymakers to revise related environmental regulations to protect public health.