Abstract: The crack defect at the welding interface between the titanium alloy skin and Wu-Cu alloy balance weight block during vacuum brazing of aircraft rudder was analyzed. The sections of the crack was analyzed by metallographic structure, fracture appearance, energy dispersion spectrum and XRD analyses. The results show that the Cu-Ti reaction occurs at the welded joint during the vacuum brazing at 950℃ and brittle compounds of multiple components are formed. The Cu-Ti reaction at the brazing interface induces erosion defects under the skin, and the crack appears at the defect under the welding stress or external force, finally extends to the surface. The oblique angle ultrasonic probe is proposed to realize the nondestructive detection of crack at the erosion area under the rudder skin.