Ambient excitation modal tests of one-way SIMO(single input multiple output)method and two-way MIMO(multiple input multiple output)method were carried out to obtain the dynamic characteristics of aqueduct bent structures. Longitudinal, transverse and bidirectional modal parameters were identified by enhanced frequency domain decomposition. The results show that most spectrum peaks of longitudinal modes are obvious with better identification results, and the longitudinal modes are more abundant than transverse ones by SIMO method. Most spectrum peaks of longitudinal or mainly longitudinal modes are also obvious by MIMO method. However, spectrum peaks of transverse modes or mainly transverse modes, bidirectional modes are small and some are difficult to be identified except for the second bidirectional order mode, i. e. the first transverse order mode. The column is identified with bending vibration. The beam is identified with rigid body vibration of translation or rotation for longitudinal low-order modal shapes, while it is characterized with bending vibration for high-order modes. For transverse modal shapes, the beam is identified with rigid body vibration of translation or immovability. The error of modal frequency identified by SIMO and MIMO methods is small. The results by SIMO method are more reliable, especially for low-order modes. Therefore, in the aspects of low-order modes identification accuracy, test workload and operation convenience, one-way SIMO method is better than two-way MIMO method for bent structure modal test.