This paper assumes the cash flow follows an arithmetic Brownian motion and discusses the capital structure that includes contingent convertible bonds ( CCB) ． We provide equilibrium prices of corporate securities and show the relationship among ruin probability，business risk and optimal capital structure． We find that CCB not only lowers ruin probability，but also decreases the risk-shifting incentive of managers and has most of the risk faced by the firm． In this way，CCB significantly increases the firm’s value and has higher yield spreads than straight bond． If a firm earns more /less whenever the market is in recession /boom，the value of the firm gets lower /higher．