The invention discloses an automatic mold release mechanism for an engine casting complex sand core mold. The automatic mold release mechanism comprises a lower core box and a mold cavity formed in the lower core box and matched with the outer contour of the lower portion of a sand core. The mold cavity of the lower core box is internally and slidably provided with a movable block in an up-down mode, and the movable block and the mold cavity enclose a cavity matched with the outer contour of the lower portion of the sand core. A jacking mold release mechanism capable of jacking the movable block and the sand core upwards is arranged on the lower part of the lower core box, and a sliding guiding structure is arranged between the lower core box and the movable block. According to the automatic mold release mechanism, the structural design is reasonable, the automatic mold release during casting can be met, efficiency is high, core-making precision is high, production cost is low, a moldis convenient to use, and the cumbersome equipment interface design is not required; and since the parts are less, the mold use and maintenance are convenient, automated core-taking production can berealized, and meanwhile, the labor intensity of operators is reduced.