Abstract: Yttrium iron garnet (YIG), as a room temperature ferrimagnetic insulator with low damping and narrow ferromagnetic resonance linewidth, has been the research hotspot in spintronics because of its spin transport properties. Bi is one of the most common doping elements used in YIG, and some researches have proved that it can tune the magnetic properties of YIG. Previous studies of Bi_xY_3–xFe₅O₁₂ thin films focused on the evolutions of their structures, morphologies, and magnetic characteristics. Yet, the effects of Bi³⁺ substitution of Y³⁺ on spin transport in YIG thin films have not been systematically studied. The regulation of YIG spin transport by doping is expected to provide a new idea for the spintronics exploration of Pt/YIG system. In this work, we prepare a series of Bi_xY_3–xFe₅O₁₂ films with different doping ratios by spin coating. And we investigate the effects of Bi³⁺ on morphology, structure and spin transport properties of YIG films. The results show that Bi doping does not change the crystal structure of YIG. The absorption of the film increases and the bandgap decreases with the increase of doping ratio. The X-ray photoelectron spectroscopy (XPS) indicates the co-existence of Bi³⁺ and Bi²⁺. The regulation of Bi doping on spin transport is reflected in the fact that the magnon diffusion length of Bi_xY_3–xFe₅O₁₂ films is significantly smaller than that of pure YIG films. Meanwhile, we find that the obvious spin Hall magnetoresistance can still be detected in the Pt/Bi_xY_3–xFe₅O₁₂ heterostructure, and the amplitude is the largest when x = 0.3.