During the retrospective cataloguing process many libraries may face the same problem;to input the items of card catalog one by one into the new sys tem of directly read the concerning items from the commercial bibliography datab ase through the retrieving point ISBN of the old circulation system in operation .If reading in the mode of batch processing,then the retrospective cataloguing i s of high speed,low cost and high quality,But the prerequisite is that the ISBN of the source and target databases have the same digits. But the ISBN of some old circulat ion system in use has a length of 10- character, the commerical bibliography database 13- character. Thus, this paper introduces the recovery calculation method to tackle this problem.