The paper develops a model of private placement, then explores the discount of private placement under the control of controlling shareholder and how to influence company's value. We find that private placement is used to a mechanism for tunneling, Whether and how much to tunnel may depend on the discount and the percentage of private placement which purchased by the controlling shareholder. At the same time, asymmetric information has an important influence on the discount of private placement, and the discount is larger when it exists more asymmetric information.