The cooperation and development of regional economy in East Asia raises a question of selection of “Fiat Money”. Economic harmonic movement index analysis indicates that East Asia should use inner fiat money and should not use Fiat Money, that US dollar, as a currency outside East Asia, should not be used as Fiat Money in regional cooperation in East Asia, that because the internationalized level of Japanese Yen is not high, furthermore, Japanese economy is in recession, etc., Japanese Yen is difficult to play Fiat Money role in East Asia, however, although Renminbi can not be regional main currency in East Asia temporarily, high correlation between Chinese economy and each economic entity in East Asia region has become important power of economic growth in each economic entity of East Asia and that the competitive pressure of China on other economic entities in East Asia is bigger than Japan and the United States, meanwhile, with gradual reform of Chinese currency exchange system, the accountability of Renminbi in East Asia region is continuously increasing. Thus, Renminbi is the most potential Fiat Money in regional cooperation of East Asia.