Based on the Acoustic Doppler Current Profiler observations in the northern South China Sea from August to November 2020, the relative volume scattering intensity was estimated using the backscattering intensity data and was used to characterize the variation of zooplankton biomass. The correlation analysis suggested that the semi-monthly variation of the relative volume scattering intensity was negatively correlated with the tidal current kinetic energy. That is, when the spring tide occurred, the tidal current kinetic energy was strong, the relative volume scattering intensity of water was low, and the zooplankton biomass was low, whereas the situation was opposite during the neap tide. Two possible mechanisms were believed to be responsible for the variation of the zooplankton biomass. On one hand, the strong tidal current during the spring tide caused the deterioration of the living environment of zooplankton and thus reduced its biomass. On the other hand, the strong tidal current during the spring tide changed the vertical migration characteristics of zooplankton, which migrated to the sea bottom, making it difficult to be observed.