The primary dendrite spacing selection in a multicomponent Ni-based superalloy during directional solidification was systematically studied using two-dimensional phase-field simulations. The alloy thermodynamic and kinetic data were obtained from Pandat software with PanNickel database and directly coupled into the multiphase-field model. All the simulations were performed on a GPU server, and an optimized computing scheme using GPU shared memory was adopted. First, the morphology of the solidification front was studied, and the segregation pattern was investigated and compared with the experimental results. Then, the dendritic spacing distribution under a wide range of pulling velocities V (p) (10-500 mu m s(-1)) and temperature gradients G (2-200 K mm(-1)) was obtained and analyzed. The simulation results agree well with analytical model that the primary dendrite spacing scales as . The coefficient b is near a constant value of 0.38 and varies slightly between 0.34 and 0.42, while coefficient c increases monotonously from 0.27 to 0.56 with the increasing G. The predicted dendritic spacing agrees well with the experimental data, but exhibits a major difference when under very low cooling rate (R < 0.1 K s(-1)). The effect of grain inclination angle theta on the final primary dendritic spacing was also studied, and an abnormal decrease in dendritic spacing was found under low grain orientation where theta < 10A degrees. When the grain inclination angle exceeds 20A degrees, the dendritic spacing increases with theta as the power law.