The magneto-optic (MO) coupled-mode equations for magnetostatic waves (MSWs) and guided optical waves (GOWs) under phase-mismatch conditions were deduced and applied to the noncollinear interaction with magnetostatic backward volume waves (MSBVWs) in obliquely magnetized yttrium-iron-garnet (YIG) film using single-element microstrip line transducer. The diffraction efficiency (DE) of the incident TE0-mode light was calculated. For the case of magnetization along the MSBVW propagation direction, the calculated results are in good agreement with the experimental data. In contrast, by using an appropriately tilted bias magnetic field the DE can be increased by 6dB and is insensitive to the phase mismatch induced by the change of magnetization direction. Thus, the optimization of the direction of the bais magnetic field is an effective method for improving the diffraction performance and increasing the MO bandwidth of the Bragg cells.