The invention discloses a preparation method of lactic acid, which takes lignocellulose as raw material, and is low in cost and high in efficiency, wherein the preparation method specifically comprises the following steps of: separating from lignocellulose to store pediococcus acidilactici, carrying out liquid culture on the pediococcus acidilactici to obtain cell bacteria taken as seeds, inoculating into a reactor with high lignocellulose content, adding cellulase, and synchronously carrying out saccharification and high-temperature fermentation to generate the lactic acid; or firstly saccharifying the cellulase of the lignocellulose to obtain sugar-containing hydrolysate, then carrying out high-temperature lactic acid fermentation by the hydrolysate to obtain the lactic acid; or uniformly blending the pediococcus acidilactici and the lignocellulose raw material, carrying out solid state cultivation, adding the pediococcus acidilactici-containing lignocellulose raw material into a fermentation tank, adding the cellulase, and synchronously carrying out saccharification and high-temperature fermentation to prepare the lactic acid. By adopting unique fermentation technology and microorganism, the preparation method is high in product concentration, simple in process, and low in cost; and particularly by taking the lignocellulose as raw material, the preparation method is wide in production space and development prospect.