For risk assessment of seawater intrusion at the Beibu gulf in southern China, numerical simulation model was adopted. According to complex river network, we carried out researches on the influence of seawater intrusion between river runoff and tidal. The results showed that, the salinity peak value was influenced mighty and reversed during the Nanliu river runoff and sea current encounter at the center of the bay. And with the gradient which altering from south to north, the salinity distribution mainly showed a great correlation between river runoff and ocean current.