Nowadays the yttrium iron gaet (Y3Fe5O12, YIG) films are widely used in the microwave and spin wave devices due to their low damping constant and long propagation distance for spin waves. However, the performances, especially the frequency stability, are seriously affected by the relaxation of the interface magnetic moments. In this study, the effect of out-of-plane magnetization depinning on the resonance frequency shift (?fr) was investigated for 3-μm YIG films grown on Gd3Ga5O12 (GGG) (111) substrates by liquid-phase epitaxy. It is revealed that the ferromagnetic resonance (FMR) and spin wave propagation exhibit a very slow relaxation with relaxation timeτeven longer than one hour under an out-of-plane exteal magnetic bias field. The?fr span of 15.15–24.70 MHz is observed in out-of-plane FMR and forward volume spin waves. Moreover, the?fr andτ depend on the magnetic field. The?fr can be attributed to that the magnetic moments break away from the pinning layer at the YIG/GGG interface. The thickness of the pinning layer is estimated to be about 9.48 nm to 15.46 nm according to the frequency shifting. These results indicate that?fr caused by the pinning layer should be addressed in the design of microwave and spin wave devices, especially in the transverse magnetic components.