The generation and propagation characteristics of magnetostatic forward volume waves (MSFVWs) in bismuth-doped YIG film under transversely nonuniform bias field is studied. The dispersive characteristics and the dynamic magnetization of the MSFVWs are calculated by using the variational approach. The influence of nonuniform field on magneto-optic(MO) coupling is analyzed. The obtained theoretical curve of the Bragg diffraction efficiency is basically agreement with the experimental results,and then it is shown that the diffraction performance for MO Bragg cells can be greatly improved by using an appropriately nonuniform bias field.