Through as an important part of service-oriented system architecture, service retrieval receives much attention from industry and academia and many methods have been proposed, there is still much to be expected of the recall, precision, efficiency and easiness of service retrieval. One important cause of this is that users cannot define their service retrieval requirements precisely and completely and this limits the function of ontology service models. Hence, an efficient service retrieval system should be able to support partial matching and help users to refine their retrieval requirements. The paper introduces a mechanism to assist users in giving high quality retrieval requirements. The goodness of the introduced mechanism to the recall, precision, efficiency and easiness of service retrieval is analyzed and its feasibility and effectivity is also verified through experiments.