This study aimed to investigate the effect of apparent temperature (AT) on hypertension outpatient visits in Haikou City and to establish a meteorological risk grade index for hypertension incidence. Based on daily data from September 2016 to April 2018, correlation analysis and a distributed lag nonlinear model were used to evaluate the association and lag effects between AT and the daily number of hypertension outpatient visits. The number of hypertension outpatient visits was significantly negatively correlated with AT. The cold effect (low AT) was predominant, exhibiting a significant lag and increasing the risk of outpatient visits, whereas the heat effect (high AT) was associated with a decreased risk. When the AT fell below 24.9°C, the risk increased as the temperature decreased; at 9.1°C, the same-day relative risk peaked (RR = 2.37, 95% CI: 1.771-3.173). When the AT exceeded 24.9°C, the risk decreased as the temperature increased; at 29.9°C, the relative risk was lowest (RR = 0.856, 95% CI: 0.741-0.989). Using the rate of increase in outpatient visits as an indicator, a risk grade index based on AT was established. The critical AT values for the high, relatively high, medium, and low risk levels were 11.07°C, 12.34°C, and 14.88°C, respectively. Subgroup analysis revealed that the effects of cold were stronger for males than for females, whereas the opposite pattern was observed for the effects of heat. The cold effect affected the juvenile group only on the day of exposure, whereas its effects on the working-age and elderly groups peaked on lag days 2 and 6, respectively. The heat effect affected only the working-age group. Low AT is an independent risk factor for hypertension outpatient visits in Haikou city, with a significant lag effect. The risk grade index based on AT provides a scientific basis for meteorological early warning and targeted prevention of hypertension in tropical regions.