A theoretical treatment on the propagation characteristics of magnetostatic forward volume wave (MSFVW), in a four-layer YIG-GGG (Yttrium iron garnet-Gadolinium gallium garnet) waveguide structure under a nonuniform bias magnetic field, and related guided-wave magnetooptic (MO) Bragg diffraction is presented. A nonuniform bias magnetic field of appropriate profile along the propagation path is found to have the effects of enhancing the power density of MSFVW and thus the corresponding MO Bragg diffraction efficiency, and to have the possibility of controlling the degree of divergence in the diffracted light.