The defect structure of passive film continues to keep mysterious and pose to challenge, especially for copper. In this paper, we applied the Density functional theory (DFT) using Heyd-Scuseria Ernzerhof (HSE) functional to explore the relationship between band structure and defect type and concentration for ultrathin passive film using Cu2O slab model. The results showed: (1) oxygen vacancies almost have no effect on band gap or conductivity; (2) V-Cu(1) induce an ultra-shallow acceptor state (0.02 eV) in band gap, which convert the original direct gap to indirect; (3) V-Cu(2) does not change the direct gap character but the acceptor energy level induced is a little higher (0.45 eV) than V-Cu(l); (4) two linear relational expressions were derived, E-g = - 5.4 * C-v + 2.18 for V-Cu(1) and E-g = - 15.6 * C-v + 2.18 for V-Cu(2), according to the variation of band gap with copper vacancy concentration. Then combining calculation results with photo-electrochemical experimental data, we concluded the approximate concentration range of defects in passive films: the compositions of passive films formed at - 0.35, - 0.23 and - 0.12 V range from Cu1.96O to Cu1.83O with 2.24% V-Cu(2) + a % V-Cu(1) (0 <= a < 6.48), Cu1.99O to Cu1.95O with 0.71% V-Cu(2) + a % V-Cu(l) (0 <= a < 2.04), and Cu1.81O to Cu1.75O with 9.44% V-Cu(l) + b % V-Cu(2) (0 <= b < 3.27), respectively.