The dynamic behaviour of a butterfly-arch stress-ribbon pedestrian bridge located in Fuzhou, Fujian, China was investigated under ambient excitation. Highly synchronous tri-axial wireless sensors were used to get an estimate of the model parameters, finding eight  stable modes with bending and torsional deformations in the frequency range 3.59-14.92 Hz. Different finite element models of the bridge were developed and a parametric optimization process with a sensitivity-based algorithm was performed to achieve the best model which could be used as baseline for a long-term monitoring of the bridge.