With the increasing demand for water transport in the Beijiang River and the expansion and upgrading of the waterway，it is planned to build a new third-line ship lock on the basis of the first-line and second-line ship locks of the Qingyuan Hub，and reserve space for the construction of fourth-line ship lock. Based on the construction scheme of the left bank of the third-line ship lock，a plane two-dimensional water flow mathematical model is established to study the navigable flow conditions and existing problems in the entrance area of the approach channel and the connecting section. The results show that：1）In working conditions 2 to 4（Q≥5 000 m3/s），the cross flow in the upstream gate area of the third-line ship lock is significant，which is greater than 0.3 m/s. 2）The flow of the Dayan River in the lower reaches of the ship lock flows into the straight channel. In working condition 3，when the flow rate of the Dayan River is 1 510 m3/s，the cross flow of the connecting section at the confluence exceeds the standard and the navigation conditions are poor.3）After further optimization test of the ship lock scheme，measures such as adding diversion piers in the upstream mouth area，arranging diversion dams and diversion piers in the downstream Dayan River mouth，and widening the dredged Dayan River mouth can effectively improve the cross flow.