The spiral selector is the key part for producing single crystal (SX) blades and ensures the integrity of crystal, which mainly includes starter block and spiral part. In this work, the influence of spiral part on the grain selection process was studied. Both of the metallography results and EBSD results proved that the prior location and the special orientation of the second dendrite arms were important for thegrains competitive growth during the directional solidification process. Based on the experimental results, two geometrical restrict mechanisms of grain selection were proposed. They were the competitive stimulating effect on the second dendrite arms in horizontal direction, which was resulted from the spiral arc shape, and the growing blocking effect on the primary dendrites in vertical direction, which was resulted from the take—off angle of the spiral part. These models could successfully explain the grain selecting effects of the spiral part. The modified cellular automaton  (MCA) technology was used to simulate the grains' competitive growth in spiral part. The changes of grains structure and orientation as the grain growing on were studied. The simulated and experimental results were compared and agreed well. Based on the simulated and experimental results, Influences of structural parameters on the grain selection behavior were proposed. The criteria for designing spiral part were also presented.