The Greater Mekong Sub-region is in the main intersection of the interests of this region. In addition to Thailand，five countries’ level of economic development is lagging behind，but the region has abundant energy and resources with a large population and huge market potential. Japan’s cooperation with the five countries of the Mekong river has been institutionalized，and the investment here also presents an increasing trend year by year. In China’s conception of the Belt and Road，the region is at the forefront and in the important hub location of the 21st Century Maritime Silk Road. China strengthens the economic cooperation with ASEAN，especially with the greater Mekong sub-region. In view of the challenges，Japan also makes policy adjustments accordingly. Around the greater Mekong sub-regional development and cooperation，competition and coordination between the two countries become the norm.