Abstract: Cross-agency synergy is frequently applied to the implement of social policy. However, the cross-agency synergy is not completely smooth, which brings about synergy failure, leading to welfare-reducing. The paper takes the placement of homeless people in S city as a case. With qualitative research method, from a perspective of cross-agency synergy, the paper introduces the synergy relationship of different departments, especially the hierarchical longitudinal synergy relationship, analyzes the phenomenon of free-riding synergy in the process of the placement of homeless people, and explains the reason of synergy failure by two analysis frameworks, institution analysis and action analysis. The paper puts forwards that Tiao-kuai relation confusion, institution supply insufficiency, path dependence, ambiguity of power and duty, and instrumental rationality action are significant reasons of synergy failure. The paper proposes the way to alleviate the cross-agency synergy failure that the government departments should establish the concept of fairness and justice, complete the institution, clarify the responsibilities of different departments, establish the cross-agency synergy mechanism and provide more comprehensive welfare for homeless people.