An effect produced by foreign banks on the credit availability of enterprise especially small firms when they entered credit markets of developing countries, is the debating point in theory circle and practice. A large number of empirical studies provided a lot of empirical research findings but without explanations. The reasons through the analyses of evolutionary games theory was found that in credit markets of small businesses, and the rational choice of foreign banks was entry while tacit permission was the rational choice of domesticsfunded banks. For the purpose of increasing credit availability of small firms, it had an important practical significance to encourage domesticsfunded banks to change location and actively participate in competition in market so as to ease financing difficulties of small business.