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Abstract: Solid-free brine completion fluids, characterized by their exceptional reservoir protection capabilities and optimal rheological behavior, are highly desirable for applications in oil and gas reservoirs and have attracted significant attention in recent decades. However, as the core component of completion fluids, the viscosifier was prone to curling or even precipitating in high-temperature, high-density inorganic salt (divalent calcium) environments, leading to failure in thickening performance. In this study, a micro-crosslinked amphoteric viscosifier (i.e., A-DDAS) resistant to high temperature and calcium ions was synthesized via free radical copolymerization of N,N-dimethylacrylamide (DMAA), diallyl dimethyl ammonium chloride solution (DMDAAC), 2-acrylamido-2-methylpropane sulfonic acid (AMPS), [2-(Methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA), and pentaerythritol triallyl ether (APE). The molecular structure and physicochemical properties of the copolymer were systematically studied by NMR, FTIR, XPS, TGA and XRD. Rheological experiments demonstrated that calcium bromide brine containing A-DDAS copolymers exhibited outstanding shear-thinning behavior and rapid thixotropic recovery, essential for efficient wellbore cleaning and fluid displacement during completion operations. As the density of calcium bromide brine increased, more calcium ions shield electrostatic attractions between the cationic and anionic moieties along the copolymer backbone, thereby promoting full extension of the polymer chains and enhancing the binding energy with water molecules. After adding 1.0 wt% A-DDAS copolymer to a 1.75 g/cm3 calcium bromide brine and aging the mixture at 180 °C for 16 h, the completion fluids exhibited an apparent viscosity of 71 mPa·s, plastic viscosity of 64 mPa·s, and yield point of 7 Pa, which were significantly better than common viscosifiers (HE300 and Dristemp). Therefore, A-DDAS copolymers demonstrated exceptional thickening capacity and dynamic shear enhancement in high-temperature, high-density calcium bromide brine, notably rendering it ideally suited for deployment in completion fluids for deep and ultra-deep wells.