Migrating legacy system with web service is an effective and economic way of reusing legacy software in a SOA environment. In this paper，we present an approach for migrating a three-tie object-oriented legacy system to SOA environment. The key issue of the approach is about services identification from large numbers of classes. And we propose a bottom-up method to model the system with UMI. And identify services from UML then. This approach can be a reference to an auto-migrating process.