Abstract: Objective To address carbon residue issues associated with traditional organic-vehicle flux pastes during aluminum alloy brazing, and the wetting failures caused by “magnesium poisoning” when using conventional potassium-based fluxes on Mg-containing aluminum alloys. Methods A binder-free, medium-temperature cesium fluoroaluminate (Cs-Al-F) flux paste with self-thixotropic properties was developed. By optimizing the reaction pH and raw material ratios during wet chemical synthesis, a composite phase of Cs2AlF5·H2O and CsAlF4·2H2O was synthesized. Free from organic solvents and thickeners, the paste exhibits excellent shear-thinning thixotropy driven by intercrystalline hydration, completely eliminating carbonized black spots during the brazing process. Results Brazing tests conducted on 6061 aluminum alloy base metals confirmed the efficacy of this cesium-based flux in resisting Mg interference, achieving a filler metal spreading area of 307 mm² at 430 °C. Mechanical evaluations revealed that joint fractures occurred exclusively within the base metal, with tensile and shear strengths reaching 134.68 MPa and 185.97 MPa, respectively. Conclusion This study offers both a theoretical foundation and a novel material solution for the precision brazing of high-value-added, complex-structured Mg-containing aluminum alloy components.