Abstract: To analyze the cause of irregular period tidal currents called "four floods and four ebbs" at the mouth of the upper North Branch in Changjiang River estuary during spring tides in dry season, the paper simulated the dynamic mechanism by means of numerical simulation and momentum analysis. The results showed that two flood currents and two ebb currents controlled by semidiurnal tide were regular period tidal currents, while the other two flood currents and two ebb currents were irregular in one day. The irregular flood currents happened with little velocity and short duration of about 2 h in a small range, meanwhile, the ebb currents in the South Branch were in the late stage. At the surface layer, the southward vertical diffusion played a decisive role, as well as the advection and the surface elevation gradient. While the baroclinic pressure gradient, which generated by the salinity front, reacted with the surface elevation gradient at the bottom layer. It indicated that the irregular flood currents at the mouth of the upper North Branch were mainly caused by the north wind stress, southward baroclinic pressure gradient forcing and the tractive action of the late ebb currents in the South Branch. The duration decreased exponentially with river discharge and disappeared when the value was greater than 22 300 m³/s. The irregular ebb currents occurred with large velocity and duration of 2.5 h in a larger area. The water mass in the South Branch spilled over into the North Branch for the earlier flood currents and formed the irregular ebb currents at the upper North Branch. The vertical diffusion, as well as the advection, did the work with the surface elevation gradient forcing from surface to bottom layers. It showed the dynamic mechanism and process of the irregular flood and ebb currents as stated above.