TC4 titanium alloy was welded with 50 steel by Nd: YAG laser. The crack propagation, microstructure and phase composition of the joint were studied. The results show that the crack type is related to the position and process parameters, and the crack propagation in the weld is transgranular, and the joint fracture is brittle fracture. The high carbon martensite formed in the weld and the middle ridge existed in the high carbon martensite. Ti-Fe phase in weld seams is TiFe, TiFe2 intermetallic compound, and the number of TiFe is higher than TiFe2. High hardness and poor toughness and plasticity of high carbon martensite and TiFe and TiFe2 intermetallic compounds are the main causes of crack initiation and propagation.