In order to enhance the security level and prevent the threats in Grid interacting with Web service environment, based on discriminating participant actors during the procedure of computation, a new threat classification method is proposed before corresponding threats model is introduced. Furthermore, we illustrate how basic security services and tools should interact with each other to provide an attack-resilient multilayer protection methodology in a service-oriented architecture. As a result, such threats analysis can be further utilized as the basis of proper security services design and operational procedures development.