Abstract: Reducing the rain-gauge Stations Method and Formula Method is more and more commonly used to design rain-gauge networks. The relationship between relative error (permissible error) of a real rainfall and Rain-gauge networks density can be determined by them. In fact, relative permissible error is relative deviation, The real permissible error is the sum of relative deviation and relative error of the most densely Rain-gauge networks, and it cannot be assessed currently. To solve this problem, a method is obtained from Levy-Lindeberg Theorem theoretically. Based on simulated rainfall fields and high-density rainfall data during Meiyu season in Anhui Province, the measurement precision of the most densely rain-gauge networks is assessed by the Method. We calculated 2005-2008 rainfall data during Meiyu season in Anhui Province and concluded that relative error of the most densely rain-gauge networks is 6%-10% in Huaibei Plain, 8%--13% in the mountains of southern Anhui Province.