The dynamic tunable polarization converter based on liquid crystals, which boastes large modulation amplitude and low power consumption, holds significant research interest in on-chip integrated systems. However, existing liquid crystal polarization converters encounter substantial transmission losses and difficulties in orienting the liquid crystal molecules. In response, this paper introduces a polarization converter based on a liquid crystal-clad slab waveguide. Initially, leveraging the anisotropy of liquid crystals and the four-layer waveguide theory, a polarization conversion analysis model that satisfied the physical characteristics of the liquid crystal-clad slab waveguide was developed. This model elucidated the internal mechanism of polarization conversion as mode coupling within the waveguide, induced by field-driven rotation of liquid crystal molecules. Subsequently, by precisely solving for the propagation constants and optical field distributions of the waveguide modes under coupled conditions, a polarization conversion analysis method suitable for liquid crystal-clad slab waveguides was proposed. This method summarized the theoretical formulas for polarization conversion efficiency and minimum conversion length, and elucidated the influence of mode coupling on polarization conversion efficiency. Finally, by comprehensively optimizing the parameters of each waveguide layer, a set of device parameters capable of achieving high polarization conversion efficiency was derived. The impact of deviations in the core layer thickness, orientation layer thickness, and electrode length on conversion efficiency was analyzed. The results indicate that the designed liquid crystal-clad slab waveguide polarization converter can achieve continuously adjustable conversion efficiency from 0% to 99.98% with a change in driving voltage of 0.2 V.