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In this paper the effects of different cooling speeds on the organization evolution of second-generation nickel-based single-crystal superalloy DD6 are revealed by high-resolution transmission electron microscopy and scanning electron microscopy. Under air-cooling conditions, a large number of secondary γ" precipitates are distributed at the matrix channels, and the secondary γ" precipitates progressively evolve in shape from spherical to cuboidal and then to butterfly-like shapes, and the size also increases with time; sharp crevices appear at the edges of the γ" precipitates and gradually evolve into serrated grooves. Under furnace-cooling conditions, a large number of fine spherical secondary γ" precipitates are distributed in the collective channel, and the width of the matrix channel increased and is positively correlated with the holding time. Under water-cooling conditions, there is no secondary γ" precipitates distribution in the matrix channel, the cubicity of γ" precipitates is complete, and there is no significant change in morphology with the extension of the holding time. γ" precipitates produces a large number of positive-negative edge-type dislocations between the two-phase interfaces in the process of selective decomposition, and the positive-negative edge-type dislocations on the two sides of the phase boundary meet to produce annihilation, reduce the surrounding energy, and promote the selective decomposition of the γ" precipitates.