The geometry, electronic structure, and optical properties of Bi and N co-doped SnO2 are investigated by first-principles calculations. The calculated results show that the N and Bi atoms can be introduced to intrinsic SnO2 with reasonable formation energy (8.95-9.61 eV/cell) at different sites. Interestingly, the BiSn15O31N presents the character of indirect gap semiconductor with n-type conductivity. Increasing the doping concentration of N or Bi, BiSn15O32-x N (x) (x = 2,3) behaves like a hole-rich semiconductor, while BiySn16-y O31N (y = 2,3) possesses the characteristic of metal. Moreover, the band gap of doped structures becomes smaller than intrinsic SnO2 due to the emergence of energy bands contributing from doping elements near the Fermi level. The absorption intensity is enhanced in UV region, and the optical absorption edge shows red-shift phenomenon for all the doped systems. Our results on Bi,N co-doped SnO2 display the improved capacity of absorption and broadened absorption region. These findings can be utilized in light sensor and solar cell.