Opening up is one of the propellers of economic development，which starts the effective allocation of initial resources，promotes the accumulation of capital and technology，and also enables enterprises to improve their own capabilities in the competition of a larger market. The “Belt and Road” Initiative is different from the existing international economic and trade cooperation mechanism，and its openness is the basic force for strengthening the economic and trade ties of the participating countries and promoting international economic cooperation and development. From the standpoint of developing countries and emerging economies，this paper analyzes the role of opening up in economic development，clarifies that participating in international economic cooperation is an effective policy choice for opening up，focuses on the analysis on the openness of the Belt and Road Initiative，and points out the positive significance of jointly building the “Belt and Road” Initiative and implementing the opening-up policy for the economic development of developing countries and emerging economies from several aspects such as power foundation，differentiated demand and endogenetic development.