According to the coupledmode theory,the effect of bias inclined magnetic field on the interaction of guided optical waves and magnetostatic waves in the Bi:YIG film waveguide is analyzed by means of the perturbation method.The diffraction efficiency(DE) and the modeconversion efficiency(MCE) in the Stokes and antiStokes interactions are calculated.Calculation results indicate that,only at normal magnetization,the MCE and the DE are equal in magnitude;under appropriately inclined magnetic field,both the MCE and the DE in the Bi:YIG film can be increased by a big margin;when the inclined angle >4,the DE (or the MCE) in the Stokes interaction is basically equal to that in the antiStokes interaction.