Aiming at the problems of the flow velocity fluctuation process caused by the unsteady flow of ship lock discharge，especially the relationships of the hydraulic factors in the restricted middle navigable channel，the transformation of unsteady flow and the corresponding scales，we study the relationship between the unsteady flow discharge process of ship lock and the flow velocity fluctuation process in the restricted middle channel through systematic experiments of physic model based on the restricted middle navigable channel of Baise navigation project. The results show that the main reasons of the generation of unsteady flow discharge wave are attributed to insufficient energy dissipation，the rapid increase of discharge during the ship lock discharge process，and the limited boundaries，which cause the flow fluctuation. By reducing the peak value of the fragmented time-discharge curve，the maximum velocity and maximum inverse velocity decrease. In the long restricted middle navigable channel，due to the energy consumption，the velocity fluctuation in the downstream is much smaller than that in the upstream，which still exists after the flow pattern reaches a steady state.