The dendritic growth behavior at the suddenly expanded cross-section of single crystal blades dramatically affects the defects formation. In this work, the dendrite growth mechanism is deeply investigated in the rectangular platform by analyzing the quasi-three-dimensional microstructures via the continuous image shooting and orientation scanning, combined with simulating the thermal fields at different furnace temperatures. As the solidification proceeds to the platform, the secondary branches at the connection between the primary zone and the expansion zone grow outwardly. Some secondary branches growth direction deflects the preferred orientation. The well-developed tertiary arms would develop on some secondary branches. The different generation dendrites with the different growth directions converge and form the boundary angles. The tertiary proportion and the dendrite boundary lengthes/angles show the decrease tendency as the temperature increases. The undercooling in the platform edges and corners from the concave/convex degree of isothermal surface should be responsible for the formation and convergence of the different generation dendrites the matrix dendrite expanding behavior towards the platform edge and their subsequent upward directional growth. The nonuniformity of thermal gradient at the isothermal surface should affects the inconsistency of the growth directions of the different dendrites, which provides the more opportunity for the dendrite deviation. This study sheds light on the dendrite growth behavior when the single crystal blade directionally solidifies through the suddenly expanded platform.