In order to reduce the consumers’ costs of renting resources in cloud computing system, a random integer linear programming model and a method for memory reservation were presented on the basis of cost constraints. Combined with the memory resource price of the reservation plan and the on-demand plan, the random cost function which consists of costs and total amount of resources constraints was designed. Aimed at minimizing the expected value of the cost function, the optimal amount of memory reserved was obtained on the basis of the probability distribution of memory consumption. The experiments show that the cost of renting recourses by utilizing this method is less than the cost of renting recourses by adopting reservation plan, on-demand plan and other similar methods.