The invention relates to a new method for producing levo-praziquantel. The method utilizes a nitrilase racemic mixture which has high stereoscopic, locus and regioselectivity and is formed through catalytic chemical synthesis for hydrolytic synthetic reaction, so that a mixture of reacted and unreacted optical isomers is obtained. The method has a mature technology and easily obtained raw materials and is low in cost. Levo-praziquantel can be produced in a simplified, optimized and large scale manner, the product purity can be over 98%, the quality standard is improved, and a foundation is laid for producing high-quality active pharmaceutical ingredients and preparations, so that the industrial problem of purifying praziquantel, which is not solved for nearly 30 years, is solved.