The institutionalization is critical for the effectiveness of social policies. The pilot project influenced by top-down political authority is an important factor in understanding the process of policy institutionalization. Based on the Neo-institutionalism theory on organizational analysis， we construct an analytical analysis framework of "Regulation-Imitation-Normalization". Taking the institutionalization process of China’s rental housing policy from 2016 to 2020 as an example， we explore the differentiated impact mechanism in policy institutionalization driven by the fiscal incentive-based pilot project and tool innovation-based pilot project. The study finds the fiscal incentive-based pilot project mainly promotes policy diffusion in time and space through regulatory mechanisms formed by implicit pressure and direct incentives, and local governments in this context often adopt pilot policies driven by institutional supply. In addition to the direct role of the regulatory mechanism, the tool innovation-based pilot project also increases the horizontal pressure perceived by local governments through the imitative mechanism, thus indirectly promotes the policy diffusion. Moreover, under the influence of normative mechanisms, the tool innovation-based pilot project shifts the driving force behind local government policy adoption from institutional supply to institutional demand. This research shows that pilot projects can influence the institutionalization process of social policies by promoting widespread diffusion of policies and stimulating the initiative of local governments, but different types of pilot projects have distinct degrees and mechanisms of influence on the institutionalization process. And the central government can flexibly select different types of pilot projects and adjust the intensity of incentives or mandates according to the evolving policy context. This flexibility is an important characteristic of China's policy pilot system.