High quality (Q) factor, tunable unidirectional guided resonances (UGRs) based on a silicon-on-lithium niobate (Si-on-LN) photonic crystal (PhC) slab are proposed and numerically investigated. The Q factors of UGRs decay quadratically with respect to the distance from the Γ point to the wave vector along the Γ-X direction, and high Q factor UGRs are obtained by moving UGR close to the Γ point. Also, a Gires-Tournois interferometer (GTI) based on a UGR with a Q factor of 9465 is numerically demonstrated, which produces a maximum group delay of 30 ps around 1.55 μm with unitary reflectance. The group delay and operation wavelengths of the GTI can be adjusted effectively by tuning the refractive index of lithium niobate (LN) and the periods of the silicon bars. Our study may find applications in PhC surface-emitting lasers, dispersion compensation, and compression of light pulses.