Abstract: The TC4 titanium alloy parts produced by laser 3D printing, cracked from the initial printing layer. The main causes of cracks were investigated by analyzing the microscopic characteristics of the cracks and the residual stress of the parts. The results show that the heat input of laser 3D printing with characteristics of high speed, micro-bedded and micro-zone continuous melting was small, and the annealing effect of the printing layer on the forming layer was feeble, which resulted in a very high residual tensile stress on surface of the printed parts and the maximum stress value even reached 640 MPa. Meanwhile, the defects such as splash, undercut and molten bead discontinuity formed during 3D printing, would affect the uniformity of thickness and density of the next layer powders, which led to the generation of micro defects and the mechanical heterogeneity of the printing parts. The micro analysis shows that the cracks originated from and terminated at the micro-defects and the cracks would develop toward the micro-defects. So, the low preheating temperature, high residual tensile stress, more micro-defects and the mechanical heterogeneity played the key roles for the root cracks in the TC4 titanium alloy parts produced by laser 3D printing.