According to the relationship between earnings and risk, and between the return on equity （ROE） and the return on investment （ROI）, this paper classifies the firms according to scales and establishes scale series econometric model. On the common basis it discusses the approximating distributions of the ROI and stochastic disturbance variable, and afterwards gets the distribution of the ROE about all the firms. Furthermore, based on the trade of money-raisers to the risks and retum, it regards the variance of the ROE as the risk measurement of raising capital, and establishes the firm＇s utility function about the ROE. Finally, through maximizing its expected utility it obtains the optimal capital structure