NOVELTY - Combined production of biodiesel and lactic acid, comprises: (a) mixing and pre-heating oil and short chain alcohol, and performing solid alkali catalyst ester exchange reaction, and phase splitting to obtain roughcast biodiesel, byproduct glycerin and solid alkali catalyst; (b) washing and vacuum rectifying roughcast biodiesel and byproduct glycerin and reclaiming short chain alcohol and roasting sodium silicate; (c) mixing resulted byproduct glycerin with water and adding roasting sodium silicate and mixing; and (d) adjusting pH of reaction liquid and replacing reacted product.    USE - The method is useful for producing biodiesel and lactic acid.    ADVANTAGE - The method: realizes the recycle of activity dip catalyst and the development of high value-added byproduct glycerin; provides economic and feasible process for the joint production of the biodiesel and lactic acid; and has a simple preparation process and reduces the production cost of the biodiesel.    DETAILED DESCRIPTION - Combined production of biodiesel and lactic acid, comprises: (a) mixing and pre-heating oil and short chain alcohol by alcohol-oil with mole ratio of 6-15 to 1 at 60-90 degrees C, then adding into a reactor to perform solid alkali catalyst ester exchange reaction for 60-120 minutes, then phase splitting and obtaining roughcast biodiesel, byproduct glycerin and solid alkali catalyst, where the short chain alcohol is methyl alcohol, ethanol, propyl alcohol or butyl alcohol, the oil is colza oil, animal oil, palm oil, Jatropha curcas oil, Pistacia chinensis oil, microalgae oil, acid-removing illegal cooking oil or low acid value bone oil, the moisture content of the oil is 0-4%, the solid alkali catalyst is roasting sodium silicate, the dosage of the catalyst is 1-10% of the quality of the oil, and the reactor is a stirring tank reactor or a padding tower reactor; (b) washing and vacuum rectifying roughcast biodiesel to obtain the biodiesel refined products, then vacuum rectifying the byproduct glycerin and reclaiming short chain alcohol and roasting sodium silicate; (c) mixing byproduct glycerin obtained by reclaiming short chain alcohol and roasting sodium silicate with water to form glycerin aqueous solution of 0.1-1 mol/l, and adding the roasting sodium silicate and mixing uniformly, where the dosage of the roasting sodium silicate is 0.5-1.5 mol/l, placing the mixture in a rapid heating agitated reactor, reacting at 200-350 degrees C for 30-120 minutes; and (d) adjusting pH of the reaction liquid to 2-3 by sulfuric acid, phosphoric acid, hydrogen nitrate or hydrochloric acid, and replacing the reacted product lactate to lactic acid, which is used as fertilizer composition or building raw materials.