Abstract: Modaokou entrance is the largest among the eight entrances of the Pearl River estuary in terms of flood discharge. Two branches (east and west) and a central sandbar outside of Modaokou entrance are rather stable. The discharge ratios of the two branches are crucial to the estuary regulation of Modaokou entrance. According to the synchronous hydrologic data obtained during the spring tide from September 10th to 11th of 2015 at outside of Modaokou entrance and anabranch, we analyzed the characteristics of measured flow velocities, flow directions and discharge ratios. The results showed that reciprocating flow is the major form of river flows for the two branches, and rotary current appears sometimes during the turning of the tide. During ebb tide, surface flow velocity is high while the bottom is low; during rising tide, flow velocity is high at the bottom and low on the surface, and a temporary phenomenon of "velocity on surface increases while decreases at the bottom" appear at the turning of tide. Discharge ratio in west branch is larger than that of the east branch during ebb tide with net discharge ratio of 66%, which is the main drainage channel of Modaokou estuary to the sea; discharge ratio in the east branch is larger than that of the west branch during rising tide and the east branch is the main tidal channel. In order to ensure the flood discharging and tide inflowing abilities of anabranch, the anabranch evolution processes and discharge ratios should be taken into consideration when developing plans for the two branches.