Abstract: The high performance of real time database system（RTDBS） requires that the main memory database（MMDB） be the base for it. There is no data I/O during the execution of transactions in MMDB. This makes it necessary for us to pre analyze the transactions and then to swap the data between main memory and external storage properly. In this paper, the strategy and method for data swapping in MMDB are discussed, and a standard to evaluate the swapping strategies is brought forward.