Due to the diversity and complexity of the design example of CNC lathe, it is difficult to obtain the relevant knowledge in the process of retrieval and modification. In this case, combined with the characteristics of the multi layer structure of CNC machine tools, similarity and genetic algorithm were used to retrieve and modify the case. A multi level structure of the CNC lathe example library was created;through the calculation of the similarity of examples,the example of a high degree of similarity with the target instance was retrieved.Then the genetic algorithm was used to modify the instance,and the similar instances were retrieved as the initial population of genetic algorithm.Through crossover and mutation, the excellent individuals were selected to participate in the genetic operation. Finally, the CNC lathe bed was taken as an example, through programming the optimal design scheme for genetic algorithm was gotten.The result shows that this method can be used to quickly find a high degree of similarity with the design problem.