In this work,we present a new mechanism for designing phase-gradient metasurfaces (PGMs) to control an electromagnetic wavefront with high efficiency.Specifically,we design a transmission-type PGM,formed by a periodic subwavelength metallic slit array filled with identical dielectrics of different heights.It is found that when Fabry-Pérot (FP) resonances occur locally inside the dielectric regions,in addition to the common phenomenon of complete transmission,the transmitted phase differences between two adjacent slits are exactly the same,being a nonzero constant.These local FP resonances ensure total phase shift across a supercell,fully covering a range of 0 to 2π,satisfying the design requirements of PGMs.Further research reveals that,due to local FP resonances,there is a one-to-one correspondence between the phase difference and the permittivity of the filled dielectric.A similar approach can be extended to the reflection-type case and other wavefront transformations,creating new opportunities for wave manipulation.