The traffic capacity of ship lock relates to daily average times of ships passing through a waterway lock, once lock tonnage, days of open to navigation per year, ship loading coefficient and un-equilibrium coefficient of traffic capacity. These factors have some uncertainties, which have an important influence to traffic capacity of ship lock. In order to eliminate these uncertainties, it uses MC computer simulation to study the traffic capacity of ship lock, and takes the new Xiagang river as an example. The result shows that computer simulation is good at dealing with uncertainties problems, and can provide a theoretical basis for construction of ship lock.