Contact-reaction brazing is used to bond AZ31 joints in order to improve the microstructure and wettability of AZ31 magnesium alloy brazed joints. This is achieved by using an AgCuZn interlayer during the brazing process without the use of a flux. The results show that a defect-free brazed joint can be obtained when the AgCuZn interlayer is completely dissolved at 500 °C for 15 minutes. The microstructure in the brazing seam consisted of multiple eutectic structures. The second phase was composed of granular and short rod-like features. Both the granular and the short rod-like structures were uniformly distributed in the continuous matrix. A small amount of primary crystal taking on cloud state is present in the brazing seam center. X-ray diffraction and energy spectrum analysis indicate that the second phase consisted of an α-Mg solid solution containing high amounts of Ag, Cu, and Zn, while the basal body consisted of an AgMg3 compound and small amounts of CuMgZn compound. The mean shear strength of the brazed joints was about 108.9 MPa. Both sugar granules and cleavage steps were clearly observed in the tensile shear fracture morphologies with mixed fracture characteristics in both intergranular fractures and transgranular fractures. The AgCuZn-base interlayer influenced the microstructure and mechanical properties of the brazed joints.