The detection of changes in the dynamic behavior of structures is an important issue in structural safety assessment. Deployment and servicing of marine and coastal structures such as piers in the marine environment with constantly changing, requires understanding the dynamic behavior of these structures to prevent possible damage. Among the factors of uncertainty in understanding the dynamic performance of piers is uncertainties related to semi-rigid connection of deck to piles. According to this fact that the main mass of the structure is on deck, the connection of deck to piles is very important. In this study, experimental and numerical model of beach piers were studied. A Test on experimental modal analysis was performed to determine the response of structures. A numerical model of the structure prepared and theory of modal analysis was performed on it. Then, based on the finite element model updating of structure approach, identify and determine the percentages of semi-rigid connections. Results show this fact the connection isn’t fully rigid. According to the present method can be compared to determine the percentage of semi-rigid connections and prepare the finite element model with more adaptable to the experimental model. Updated results with this method were very close to the real model.