Assessing urban thermal equity at the neighborhood level requires integrating physical exposure, demographic vulnerability, and adaptive capacity, yet many cities lack the longitudinal demographic data that established vulnerability frameworks assume. This paper presents the Thermal Equity and Population Index (TEPI), a diagnostic framework designed specifically for data-constrained contexts where only a single census cross-section, satellite-derived thermal variables, and current service-facility records are available. TEPI combines three dimensions—thermal exposure from Landsat imagery, population vulnerability from census age structure, and adaptive capacity from point-of-interest data—through a multiplicative composite that flags streets where multiple disadvantages co-occur. To evaluate the framework’s properties, we apply TEPI to 269 street-level units in Beijing and 74 in Shenzhen across three observation years (2015, 2020, and 2025), test its structural sensitivity against three alternative aggregation schemes, decompose its internal composition through OLS and GWR, and assess its cross-city transferability using 108 streets in Shanghai. Rank correlations between the baseline multiplicative specification and the alternative formulations exceed 0.97, and quintile overlap for the highest-burden streets ranges from 0.84 to 1.00, indicating that priority identification is robust to aggregation design. The transferability test shows strong ranking consistency (Pearson r > 0.90) but limited absolute portability (transferred R² = 0.22–0.26), establishing that TEPI is better suited as a comparative screening tool than as a plug-in prediction model. The framework also reveals a substantive pattern: in Beijing, mean TEPI declined over the decade while the Gini coefficient rose from 0.26 to 0.31 and Moran’s I increased from 0.29 to 0.39, demonstrating that average improvement in thermal equity can coexist with widening spatial inequality. TEPI provides a transparent, replicable screening tool for street-level thermal equity assessment in cities where comprehensive longitudinal social data are unavailable, while its documented structural and transferability properties help practitioners judge where the framework can and cannot be applied.