The pipe flange is made of nickel-plated stainless steel tube and aluminum pipe welded by brazing. In the mechanical test process, the weld seam cracks and leads to leakage. Through morphological observation,energy spectrum analysis and metallographic analysis it is considered that 10 to 20 μm Al-Fe intermetallic layer is found in weld interface,flange welds cracking reason should be the weld interface in the welding process of over reaction,which generates brittle metal thick intermetallic layer, and brittle fracture occurred in the welding joint during the mechanical test.