Abstract: The butt brazing tests at 930 ℃for 30 min of TC4 titanium alloy were carried out by using B-Ti57CuZrNi solder under different brazing gaps. The effects of the brazing gap on the microstructure and high temperature mechanical properties of the joints were investigated.The results show that the joint defects increase with the increase of the brazing gap.The good weld filling with no obvious defects is observed when the brazing gap is less than 0.1 mm.The microstructure of the brazing joint presents the characteristic of cast structure when the brazing gap is of 0.06-0.30 mm, and the diffusion layer with thickness bigger than 30 μm exists at the original interface, and effective metallurgical bonding is obtained.With the increase of the brazing gap, the width of the brazing seam increases, the cast microstructure of the brazing joint is more obvious, and the high temperature tensile strength decreases. When the brazing gap is bigger than 0.1mm, the high temperature tensile fracture of the brazed joint occurs in the weld. The high temperature tensile strength of the joint is lower than that of the matrix, and the fracture belongs to brittle fracture. When the brazing gap is less than 0.1mm, the high temperature tensile strength of the joint is higher than that of the matrix.Considering from obtaining good 400 ℃tensile properties of the brazed joint, the brazing gap should be less than 0.1mm.