Abstract: There must be errors for the measuring values of the first 1 order data of a system due to the noise and other factors in the Testing Model Analysis. The errors must affect the calculating accuracy of a and 3. To improve accuracy, as a theoretical discussion, this paper presents a new calculating method-the Weight Least Square Method. This method could get more accurate initial value of a and 3 when dynamical respond of construction is calculated by the Method of Finite Element.