Intrinsic defects and hydrogen play an essential role in regulating photocatalytic properties of semiconductor materials. Bismuth oxychloride (BiOCl) is an important ternary bismuth-based photocatalytic material, for its wide bandgap allows band-gap engineering to tailor its properties. In this paper, we use first-principles to study the effects of intrinsic defects and hydrogen on the electronic structure of BiOCl. We studied the intrinsic defect formation energies, defect electronic structures and the diffusion of these defects in BiOCl. We found that the p-type conductivity and band gap variation in BiOCl may be caused by the Fermi level pinning effect and intrinsic defective states around the band edges. Intrinsic defects of Hint, OCl and BiCl show delocalized features which may have a predominant effect for carrier transfer. As the temperature rises, the diffusion of oxygen vacancies in the xy plane goes up most drastically, while the diffusion of interstitial hydrogen in the yz plane changes smoothest and has the fastest speed. Our work offers fundamental insights for explaining the band gap variation, p-type conductivity and the transport of defects in BiOCl and provides basic guidance in regulating the photocatalytic performance of BiOX (X = Cl; Br; I) by defect engineering.