The grain selection process during single-crystal casting of a Ni-base superalloy DD5 in a spiral grain selector was simulated by a macroscale ProCAST coupled with a mesoscale cellular automaton finite element (CAFE) model, and the simulation results were validated by experimental observations. The results show that at the same withdrawal rate, the number of dendrites decreases gradually, and the primary dendrite arm spacing increases gradually with the increase of the distance from the chill surface. With increasing the withdrawal rate from 2 mm/min to 8 mm/min, the primary dendrite arm spacing decreases, the number of dendrite stems increases, the dendrite spacing decreases and the dendrite structure is refined gradually. During the simulation process, the quantity, area, and color of the solid tissue are similar at the three withdrawal rates of the spiral selector starter block. In the spiral part, the microstructures at 5 mm/min are less than those at the other two withdrawal rates, with larger area and lighter color. The grain selection efficiency at the intermediate withdrawal rate is better than that at the other two withdrawal rates, which is obtained from the results of the grain microstructure evolution and metallographic microstructure analysis of crystal selector.