Routine hypertension management primarily targets acute cardiovascular events, frequently overlooking progressive physiological decline (frailty) and its economic implications. This study aimed to identify systolic blood pressure (SBP) trajectories and evaluate the mediating role of frailty in their association with catastrophic health expenditure (CHE) among middle-aged and older adults.

We analyzed 3996 participants from the China Health and Retirement Longitudinal Study (CHARLS). Group-based trajectory modeling identified SBP evolution patterns (2011–2015). The 2015 Frailty Index (FI), based on a 27-item deficit accumulation model, served as the mediator. The primary outcome was CHE in 2018, defined as annual household healthcare expenditure exceeding 40% of the household’s capacity to pay. Mediation analysis quantified total, direct, and indirect effects.

Three SBP trajectories emerged: “Low-stable” (50.0%), “Moderate-increasing” (41.3%), and “High-decreasing” (8.7%). Compared with the Low-stable group, the High-decreasing trajectory was significantly associated with higher 2015 FI scores ( β = 0.029, P < 0.001). A statistically significant indirect effect through frailty was observed for the association between the High-decreasing trajectory and CHE in 2018 (ACME = 0.0084, 95% CI: 0.0031–0.0100, P < 0.001), although the direct and total effects were not statistically significant. Findings were similar in sensitivity analyses using alternative CHE definitions and BMI trajectories.

Frailty may be one pathway linking the High-decreasing SBP trajectory with subsequent household CHE. These observational findings support considering functional health alongside blood pressure in middle-aged and older adults. Further prospective studies are needed to clarify the temporal and causal relationships and to assess whether incorporating frailty assessments into cardiovascular management improves clinical and socioeconomic outcomes.