Abstract: The thread of CuZn39Pb3 lead brass joint cracked during the sealing test. The causes of joint cracking were analyzed by means of macroscopic observation, scanning electron microscope analysis, metallographic examination and energy spectrum analysis. The results show that a large amount of lead concentrated in the grain boundary, weakening the grain boundary strength and reducing the tensile strength and plasticity of the material. When the machining residual stress and assembly stress exceeded the tolerance limit of part of the grain boundary of the thread, microcracks would appear at the thread, and the microcracks would eventually lead to the cracking of the lead brass joint.