the invention relates to a new method for preparing - praziquantel . in the invention , by taking advantage of the high stereo selectivity , site selectivity and region selectivity of an enzyme , an intermediate of a pure optical and chiral - praziquantel are obtained by means of the dynamic kinetic resolution of an enantiomer from the synthesized racemate or derivatives thereof , and the - praziquantel is obtained by using various conventional and mature organic chemical reactions with higher yield . the method of the invention has the potential advantages of easily available raw materials , low cost , environmentally safer process and convenience for large - scale production . also , the purity of the end product can be more than 98 %. by adopting the invention , the quality of the product is improved and a basis for developing high quality of active pharmaceutical ingredients and formulations is established , and thus the pending industrial problem of purifying praziquantel over 30 years becomes solvable .