Abstract: The basic equations describing unsteady now in complicated manifold filling and emptying system are derived. The numerical method used to solve these equations is based on backward differences, and difference equations are in implicit form. Compared with the laboratory data, the solution by computer is in agreement with the observed. By means of numerical calculation, the distribution of flow in a lock chamber manifold is investigated.The results show that edge-shape of lock chamber ports, culvert length and head loss in culvert will all affect the distribution of port discharge.