Abstract: Objective It aims 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. For butt joints, fracture occurred in the weld zone, whereas for lap joints, fracture occurred in the base metal, with tensile strength and shear strength reaching 134.68 MPa and 185.97 MPa, respectively. Conclusion 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.