The vibration characteristics of the storm-safe drilling riser after disconnecting are related to the actual production safety. In this paper, the vibration response in this special condition is analyzed by the model test. The results show that after disconnecting, due to the release of the upper end of the riser, the vibration mode of the riser performs like the elastic support rigid body dose when the Reynolds number is low. With the increase in Reynolds number, the self-excited vibration of the elastic support rigid body and the modal vibration of the flexible tube simultaneously exist in this special condition presenting from the primary and secondary frequency. With the further increase in Reynolds number, the status between the primary and secondary frequency exchanges which corresponds to the transformation of the vibration mode. The transformation of the vibration mode in in-line direction and cross-flow direction are not synchronous, that in the in-line direction requires a larger flow rate. Compared with the connection state, the vibration mode distribution trends to move up to the top of the riser after disconnecting.