In order to obtain a high gain, a Huygens' metasurface was proposed as the phase shift surface (PSS) for the resonant cavity antenna (RCA) to uniformize the field phase distribution over the RCA's aperture. The metasurface consists of two sets of split ring resonators (SRRs), which are parallel and perpendicular to the incident electromagnetic wave, respectively. The SRRs resonate with the magnetic and electric field of the incident wave, and thus the metasurface is able to manipulate the reflection and transmission of the incident wave. By configuring the dimensions of the SRRs independently, the magnetic and electric resonances can be adjusted respectively, and thereby a fast design of the PSS is achieved. To verify the proposed method, a PSS sample with an operating frequency of 5.8 GHz was designed and applied to a cylindrical RCA with a diameter of 180 mm. The RCA with the PSS was fabricated and the measurement agrees will with the simulation. The simulated and measured results demonstrated that the PSS had a phase shift range of 360° with a minimum transmission magnitude of 0.99 and the RCA with the PSS obtained a more uniform field phase distribution over the antenna's aperture. The RCA's gain was enhanced from 14.90 dB to 18.31 dB, and the aperture efficiency was enhanced from 25.9% to 56.7%.