Abstract: Emergency supplies have the characteristics of large peak demand, strong demand uncertainty and high shortage cost.Therefore, a certain amount of emergency supplies must be reserved by governments in advance.However, if there is no demand within period for a certain kind of supplies with a quality guarantee period, such as food and medicine, it will not only cause a lot of waste, but also increase the financial burden of governments. Although surplus supplies can be sold back to the supplier by the government through buyback contracts, which can reduce the government's cost, but additional benefits is not gained by the supplier with taking on excess risks. Based on this, a procurement model of emergency supplies based on put option contracts is designed, which is used to solve the problem. After solving the optimal decision of the government under the put option contract, some conclusions are found:
(1) Only when the option price and execution price of the put option contract satisfy a certain condition, the put option contract can be used by the governments to purchase supplies, and the conclusion that the buyback contracts is a special form of the put option contracts is also found. In addition, compared with the traditional purchase mode (based on the wholesale price contract), the risk of the quality guarantee period can be solve effectively by the put option contract, which can also increase the government's purchasing volume and effectively reduce the shortage risk.
(2) On this basis, the mechanism of realizing supply chain coordination between the government and the enterprise is analyzed and the conclusion that there is a negative correlation between the option price and the execution price, which is different from supply chain coordination conditions under call option contracts, is also found.Meanwhile, both the government's cost and the supplier's income increase with the increase of the option price. Therefore, under the put option contract, the government will prefer low option price, while the supplier will prefer high.
(3) Finally, compared with the traditional procurement model, the range of the option price is given when the government and the enterprise achieve win-win cooperation under the put option contract. What's more, compared with the buyback contract, the conclusion that the purchase mechanism based on the put option contract can reasonably compensate the loss caused by the supplier assuming surplus risk and reduce the government's cost is proved, which reflect the superiority of the procurement model of emergency supplies based on the put option contract.

Key words: emergency supplies purchasing, government reserve, put option contract, quality guarantee period, coordination of supply chain