Abstract: The research on rapid composition approach for large scale services is still a critical challenge.Exploring service composition satisfiability and service deposit optimization is a useful attempt for such issues.By introducing the concept of Web service request input closure, the paper demonstrates a method of determining service request satisfiability and optimizing Web service deposit, and analyses the effect about the ratio of the total number of field properties and Web services to user request satisfiability and Web service deposit optimization by experiments.The concept of minimal user input cost is put forward and a greedy algorithm to calculate the approximate minimum cost input set is proposed.