The thin-wall structure of the second generation single-crystal superalloy DD405 was fabricated by laser directed energy deposition technology. The mechanism of hot crack formation in the process of laser directed energy deposition was analyzed and explored by combining experiment and theory. The formation of hot cracking was determined by stress concentration, liquid film stability and carbide precipitated phase. The residual stress in deposition region increased with the deposition height due to the layer-by-layer stacking of the laser directed energy deposition process, so there is a high-level tensile stress in the deposition region. Significant stress concentration occurred at the grain boundary of the deposition region, and the liquid film was teared under the tensile stress on both sides, leading to crack initiation. The stability of liquid film was closely related to the angle of grain boundary between adjacent grains. When the high angle grain boundary is larger than 40°, thermal cracks will be formed under the drive of tensile stress. MC-type carbide precipitates promoted crack initiation by a pinning effect on the liquid feed and weakening interface bonding strength with the substrate.