In 2015 three pieces of legislation were passed, which lays down a set of legal foundations for Taiwan's spatial planning. To shed light on the decision making of the relevant legislation, this article focuses on the congressional agenda setting, in which sponsoring and cosponsoring networks among legislators are analyzed to clarify the factors affecting legislators' behaviors and interactions. In response to the policy characteristics of national spatial planning, this paper applies social network analysis with its ＂actor-relation＂ concepts to examine four driving factors that most likely underlie the decisions of legislators, including interest, party, cognition and network structure. The results of the study reveal that legislative collaborative actions in ＂sponsoring-cosponsoring networks＂ are more likely to occur when an integration of local interests emerges rather than a call for common interest. While a cross-party coalition does not appear in the sponsoring-cosponsoring networks, intraparty cooperation dominates the interactions of bill support. It is founded that the sponsors possess more extended ＂adaptation＂ cognition, the seniority system of committee have strengthened legislative professionalism, and ＂reciprocity＂ and ＂transitivity＂ are both dominant types of subgroups within the networks, which all forge the driving forces toward collective actions for the agenda setting. These research findings deepen our knowledge of the relevant legislative decision-making and are deemed to facilitate future research and practice regarding Taiwanese spatial planning and land use.