The rapid development of advanced aero-engine and industry gas turbine requires high performance of single crystal (SX) blade. Spiral selector is very important to produce SX blade, which includes starter block and spiral part. In this research, grain density changing and orientation deviating as the height of grain growth (the distance between section studied and the undersurface of the sample) increasing were studied by the experiment and simulation, and the designing rules for the starter block were given out. EBSD orientation mapping technology was used to get grains’ morphology and orientation. Mathematical and physical models were built for the directional solidification process. Adopting CA-FD method, the 3D macro temperature field of solidification process was calculated as well as grain growth. The properties of grains competitive growth and evolution process during directional solidification in starter block were analyzed based on macro and micro modeling results, and rules for grains competitive growth was explained, which provided theoretical supports for designing starter block.