Abstract: The real estate developers'breaking of contract brings about a serious loss to the agents during the process of real estate agents sales. It is the key to solve this problem by determining the amount of penalty signed by both sides in the contract accurately. In this paper, based on the principal-agent theory and the practical problems, the penalty calculation model was established by increasing the developer's profit margin bound. And through the analysis, the amount of penalty was directly proportional to the opportunity costs, marketing costs and the risk costs of agents; the profit margin was inversely proportional to the expectation profit margin of developers. Ultimately the penalty range was determined. And then a calculation model was tested through a numerical example. Establishment of the penalty calculation model provided a basis to the sale agents in the signing of the contract to ensure their own interests, at the same time, a "win-win"situation could be reached in the process of cooperation.