Thin walls of an AZ91 magnesium alloy with fine equiaxed grains were fabricated via cold arc-based wire arc additive manufacturing (CA-WAAM), and the droplet transfer behaviours, microstructures, and mechanical properties were investigated. The results showed that the cold arc process reduced splashing at the moment of liquid bridge breakage and effectively shortened the droplet transfer period. The microstructures of the deposited samples exhibited layered characteristics with alternating distributions of coarse and fine grains. During layer-by-layer deposition, the β-phase precipitated and grew preferentially along grain boundaries, while the fine η-Al₈Mn₅ phase was dispersed in the α-Mg matrix. The mechanical properties of the CA-WAAM deposited sample showed isotropic characteristics. The ultimate tensile strength and elongation in the building direction (BD) were 282.7 MPa and 14.2%, respectively. The microhardness values of the deposited parts were relatively uniform, with an average value of HV 69.6.