Various semantic web service discovery techniques have been proposed, many of which perform the profile based service signature (I/O) matching. However, the service I/O concepts are not sufficient to discover web services accurately. This paper presents a new method to enhance the semantic description of semantic web service by using the semantic constraints of service I/O concepts in specific context. The semantic constraints described in a constraint graph are extracted automatically from the parsing results of the service description text by a set of heuristic rules. The corresponding semantic web service matchmaker performs not only the profile’s semantic matching but also the matching of their semantic constraints with the help of a constraint graph based matchmaking algorithm. The experiment results are encouraging when applying the semantic constraint to discover semantic web services on the service retrieval test collection OWLS-TC v2.