Abstract: Nickel-based single-crystal (SX) superalloys are the key metallic materials of aeroengines. However, thermomechanical deformation always occurs during the directional solidification of SX superalloys, negatively influencing the SX structure. Casting deformation is simulated in most of the previous studies, whereas the direct simulation of dendritic thermomechanical deformation has been largely ignored, resulting in a lack of comprehensive understanding of this process. In this study, we systematically investigate dendritic thermomechanical deformation with a model coupled with dendrite growth, fluid flow, and thermomechanical deformation behavior. Results reveal that the dendritic thermomechanical deformation-induced dendrite bending is not randomly distributed but is mainly concentrated on the casting surface. The dendritic thermal stress increases as dendrite grows and accumulates after dendrite bridging. Transverse thermal contraction mainly occurs at the edge of casting in the corner, and axial thermal contraction is larger than transverse contraction. The high-stress region of the primary dendrite trunk is mainly distributed below the dendrite bridging near the solidified part, and the stress along the transverse direction reaches its maximum value on the casting surface. Stress concentrated on the casting surface is mainly attributed to variations in transverse temperature gradients caused by heat dissipation on the lateral mold wall, and inconsistent constraints in the lateral mold walls.