To assess the equity performance of an urban rail-transit-oriented public transport system and identify potential issues, this study uses mobile phone signaling data from Shenzhen to build a job accessibility equity evaluation framework integrating spatial and group equity, and proposes a "spatial-disparity identification first, population-characteristic summarization second" approach. Absolute and relative job accessibility are jointly measured under different commuting scenarios; transport-disadvantaged units are identified using the "dual-disadvantage" criterion of low absolute and high relative accessibility; K-medoids clustering is used to characterize socioeconomic attributes; and a decomposable Theil index is applied to identify sources of inequity. Results show a distinctive pattern in Shenzhen's job accessibility: unequal spatial access to job opportunities alongside convergent commuting time. Rail transit shortens commuting time, but job opportunities remain concentrated in station core areas and locationally advantaged areas, while transport-disadvantaged units increase with distance from rail stations. Four disadvantaged groups are identified—multidimensionally disadvantaged groups, experience-based labor groups, traditional labor groups, and high-income, high-education commuting-mismatch groups—revealing diverse mechanisms of disadvantage. Inequity is driven mainly by within-group disparities, while between-group differences are largely driven by education and income. The study provides new insights for equity measurement and sustainable operation of urban rail transit-oriented public transport systems.