On January 16, 2016, AIIB declared its opening for business. Ever since its announcement by Chinese leaders in 2013, AIIB has been warmly embraced by most countries of the world. But still, neither the US nor Japan have become the AIIB members. Both of them take a rather cautious approach, viewing AIIB more as a challenge to the existing international financial order. By comparison, this paper finds that each country casts similar doubts. The US places its focal point more on ‘China-led’ rather than ‘new MDB.’ Japan focuses more on ‘new MDB’ instead of ‘China-led,’ meanwhile considering how to cope with the dilemma between the US and China. Orienting itself towards a more balanced international economic order, this paper attempts to seek a kind of positive cooperation based on the coordination of tri-lateral interests, thus suggesting the US and Japan to join AIIB as an external supervisor and internal member respectively.