Thin-walled specimens of the DD6 single crystal superalloy with different thicknesses were prepared using Bridgman method. The effect of section size on the microstructure of the thin-walled specimens of DD6 single crystal superalloy was investigated. The results show that with the reduction of section size, the primary dendrite arm spacing and the eutectic percent of the thin-walled specimen as-cast first decrease and then increase, and the average size of γ’ phase in the dendritic core of the thin-walled specimen after heat treatment declines gradually. The appropriate decrease in section size can reduce the degree of interdendritic segregation, but when the section size is reduced to 0.5 mm, the degree of interdendritic segregation increases