Building a unified national market is the base and internal requirement for constructing a new development system. This article built a multi-regions spatial equilibrium model including the flow of commodity-factor-resources, to quantify the welfare effects and find that the building of a unified national market can significantly increase the overall average share of inter-provincial trade, the overall average share of inter-provincial migration and the land utilization efficiency. Overall, the building of a unified market can increase utility by an average of about 6% directly, and become greater when considering the indirect effect. According to different scenarios, the higher the efficiency of the flow of commodity-factor-resources realized by building a unified national market, the higher the welfare effect. Based on different current characteristics, the welfare effect exhibits heterogeneity among different regions and provinces. For building a unified national market and making full play of the advantages of the ultra-large-scale market, suggestions are promoted from three aspects including building the unified national product market, labor market and land market.