A polarization-ratio control device based on the selective Bragg reflection of cholesteric liquid crystal (CLC) was designed for single-beam spin-exchange relaxation-free (SERF) atomic magnetometers, in which the same optical beam is used for both atomic pumping and magneto-optical detection, while conventional polarization-control optics hinder system miniaturization. An ellipticity angle of 22.5° was selected as the target operating point, corresponding to a required power ratio of 5.83:1 between the left- and right-handed circularly polarized components. This polarization requirement was converted into structural parameters of the CLC device, and a two-dimensional frequency-domain model was established in COMSOL Multiphysics for numerical simulation. The results show that, at an operating wavelength of 795 nm, the designed CLC pitch is approximately 497 nm. When the effective thickness is approximately 4.34 pitches, the reflectance of the target-handed circularly polarized component reaches approximately 82.84%, whereas the orthogonally handed component maintains a low reflectance, enabling effective control of the transmitted circular-polarization power ratio. A CLC sample was subsequently fabricated and characterized experimentally. The transmitted beam retained an approximately Gaussian spatial intensity distribution. Under small variations in the angle of incidence, the relative variations in both transmitted power and ellipticity remained within approximately 0.5%. During a 30-min continuous measurement, the transmitted power remained stable at approximately 3.68 mW, with an overall fluctuation of less than 1.72%. The experimental results demonstrate that the proposed CLC device can achieve elliptical-polarization ratio control through finite selective reflection, providing a potential approach for the miniaturization and integration of polarization-control optics in single-beam atomic magnetometers. Key words: atomic magnetometer; cholesteric liquid crystal; selective Bragg reflection; elliptical polarization; polarization ratio; polarization control;COMSOL Multiphysics software; optical-path integration