Abstract: A mathematical model is proposed on an operational bridge, and the bridge parameters are statistically estimated by vehicle responses. The data, modal set, and criterion of the operational bridge system were obtained by the principle based on the system identification theory. The indirect identification was performed by an optimized criterion. The vehicle response was reconstructed and the point estimation and the interval estimation were performed on the bridge frequency and stiffness. The influence of the different modal set was investigated by the methods base on the semi-analytical, numerical, noised semi-analytical and numerical solution. The model set affects the point estimation significantly and the noises affects estimation interval pronouncedly, which indicate that the proper set and the noises decreasing are the key factors for the engineering applying.