The invention discloses a method for preparing polylactic acid by catalyzing solid acid composite lactate, comprising the following steps of: dehydrating lactic acid monomers with different optical activities used as raw materials by decompressing; and adding pre-active resin for reaction to obtain a lactic acid performed polymer with molecular weight of 2000-3000Da; then, adding lactate with different optical activities into the lactic acid performed polymer; decompressing, melting and polymerizing to obtain a low lactic acid polymer at the temperature of 120-180 DEG C through composite catalysis; decompressing the low lactic acid polymer at the temperature of 130-180 DEG C or carrying out solid-phase polymerizing in an inert atmosphere to obtain a product with 64 percent of a crystallinity degrees (Xc). The invention uses the composite catalysis of the solid acid and the lactate, not only increases catalyst efficiency and stereoselectivity, but also reduces the pollution of inorganic acid, wherein the reutilization of the solid acid greatly reduces cost. The obtained polylactic acid has weight-average molecular weight of 120000Da, a vitrification temperature value (Tg) of 60 DEGC and a melting temperature (Tm) of 180 DEG C. The product has optical activity purity (OP) of 94 percent and has he characteristics of high molecular weight, high-melting points, high thermal stability and high optical activity.