Propagation of magnetostatic surface waves (MSSW) in yttrium-iron-garnet (YIG) film placed in external homogeneous slowly time-varying magnetic field was investigated theoretically in geometrical optics approximation. The MSSW trajectories for the cases of amplitude and angular modulation of the magnetizing field were calculated. Anisotropic MSSW propagation in oscillating magnetizing field may be used for spectral analysis of microwaves.