The invention discloses a high-heat-resistance skin-core structural polylactic acid (PLA) fiber and a preparation method thereof. The core layer of the skin-core structural PLA fiber is stereocomplex polylactic acid (scPLA), and the skin layer is PLA with the L-lactic acid structural unit (D-LA) content of 0 to 5 mol percent. The preparation method of the skin-core structural PLA fiber comprises the following steps: after raw materials are dried, adding the raw materials into corresponding hoppers of a composite spinning machine respectively, and carrying out melt conveying, spinning, blowing cooling, winding feeding, oil bath stretching, steam bath stretching, tension heat setting, oiling, stacking, curling, relaxation and cutting to prepare skin-core structural PLA short fiber; or after the raw materials are dried, adding the raw materials into the corresponding hoppers of the composite spinning machine respectively, and carrying out one step method of melt spinning, side blowing cooling, oiling, stretching and heat setting to prepare skin-core structural multifilaments. The prepared skin-core structural PLA fiber is relatively high in heat resistance, and can be used in the fields of textiles, composite materials and the like.