This invention relates to a method for preparing polylactic acid foamed plastic, especially polylactic acid foamed plastic sheet material for sucking-plasticization. The raw materials for the sheet material are: polylactic acid resin 100 parts, chain extender 0.2-2 parts, nucleating agent 0.1-5 parts, foamer 0.3-2.5 parts, toughener 0.5-15 parts, reinforcer 0.5-5 parts, and antioxidant 0.1-1 part. The method comprises: extruding the raw materials without the foamer to obtain master batch, adding the foamer, extruding, stretching unidirectionally, and shaping, cutting, spreading, coiling and packaging. The foamed plastic sheet material has good mechanical properties and obstructive ability, and is biodegradable, thus the cost performance is high. The sucking-plasticized products can be used to manufacture bottles for cosmetics, pesticides and drinks, food packaging containers, dishwares, trays and vacuum-shaped products.