The reform of treasury cash management is an important part of the construction of modern treasury system in China. The main task of deepening the reform is to establish the management system of the target of the cash balance in the treasury account. The key step is to determine the reasonable scale of the cash balance in the treasury account using scientific methods. In this paper, we analyse the Baumol model and MillerOrr model which are most commonly used currently, and propose using the Value at Risk (VaR) model in financial risk management to calculate the target of the cash balance according to practical situation. We use the net outflow data of Chinas central treasury in 20122017 to calculate the target of the cash balance in the central treasury account，and find that the number should be 120 billion yuan. At last，we put forward some policy suggestions for establishing the management system of the target of the cash balance in the treasury account.