This paper presents the theory and methodology of a relatively simple and practical modal test approach based on ambient excitation for long span bridges. In this approach

some fundamental parameters

such as mode shapes and their corresponding frequencies as well as the damping ratios

can be found by a fast process of spectra analysis of the measured response signal. Also

its results are direct and reliable. The successful practices of the modal test on Guangdong Jinma Cable-stayed Bridge shows that the method is valid.