Carboxylic sulfuric anhydrides are key aerosol constituents. Among these, benzoic sulfuric anhydride (BSA) has been predicted to reach up to 10⁷ molecules·cm^–3 in urban industrial regions and has been identified as a potential precursor for new particle formation (NPF). However, its formation mechanism and role in sulfuric acid-ammonia (SA-A) nucleation are unclear. Here, we employ quantum chemical (QC) calculations and atmospheric cluster dynamics simulations (ACDC) to investigate the gaseous mechanism of BSA formation and its key role in SA-A nucleation. QC calculations show that BSA forms via fast cycloaddition between BA and SO₃ (barrier of 1.5 kcal·mol^–1). Within 280.0-320.0 K, this route can effectively compete with the SO₃ + (H₂O)₂ reaction at [BA] > 10¹⁰ molecules·cm^–3 and RH < 60 %. ACDC simulations further reveal that at [BSA] = 10⁸ molecules·cm^–3, the SA‑A cluster formation rate increases by six orders of magnitude as temperature decreases from 298.15 K to 278.15 K. In high-BSA industrial regions (e.g., Beijing), the BSA-SA-A ternary pathway contributes 99 % to total cluster formation. Notably, despite its lower concentration, BSA exhibits stronger nucleation potential than its precursor BA in the SA-A system. These findings clarify the atmospheric sources of BSA and help explain frequent NPF events in urban-industrial regions.