The formation of low-angle grain boundaries (LABs) in the rejoined platforms of a Ni-based single crystal superalloy under different directional solidification rates was investigated by the experimental investigation and the ProCAST simulation. The results showed that the growth morphology and orientation evolution of dendrites in the platforms were different under the withdrawal rates in the range of 60–100 μm/s and then resulted in different types of LABs. At lower withdrawal rates, the longitudinal LABs were common in the rejoined platforms. Both the sliver defects and the orientation deviation of original primary dendrites from two independent growth paths could cause the longitudinal LABs in the platforms. At higher withdrawal rates, the dendrite growth patterns were more complex and the secondary branches with lateral growth tended to deviate from their original orientation, eventually leading to the formation of some transverse LABs. Finally, some suggestions to prevent the formation of different LABs are provided.