Considering the problems of low load rate of data centers in cloud providers, uncertainty and variety of cloud user demand; in order to improve the average profit of the cloud providers, an overbooking model of multiple instances under uncertain demand was established. The proposed model combined the influences of overbooking for cloud data center load balancing and Service Level Agreement (SLA) under the actual cloud computing resource market, multi-constraint of overbooking was provided, then the optimal allocation policy of each instance type was put forward. The simulation results show that when the unused rate of reservation is 0.25, the average profit is relatively high, the load rate of data center is 78%, finally the optimal allocation of each instance type is determined.