Service outsourcing has become an important strategy for enterprises to develop and one of the important instruments for generating competitive advantage in the process of value chain cooperation and competition. From the view of cooperative reciprocity among enterprises, the author tries to explain cooperation mechanism of service outsourcing by the theories of value chain, evolutionary and reciprocity. The paper further shows that service outsourcing is an important platform and impetus to cooperation development among enterprises. Based on that, the paper designs as well as researches the innovation mode of reciprocity cooperation development.