The mucus produced by the outbreak of Phaeocystis globosa in the adjacent waters of the Fangchenggang Nuclear Power Plant (FCGNPP) in China has blocked the entrance of the cooling water filtration system of the FCGNPP, and posed a threat to the safe operation of the FCGNPP. At present, there is no related research on whether the changes in seawater viscosity could be used as a new method for monitoring and providing early warning of P. globosa tide. During a complete red tide cycle, the temporal and spatial changes in the hydrological conditions (temperature, salinity, dissolved oxygen), chlorophyll- a (Chl- a), composition and abundance of phytoplankton, number and size of P. globose colonies, concentration of transparent exopolymer particles (TEP) and the seawater viscosity were measured in this study. The results indicate that there was an extremely significant negative correlation between the physical seawater viscosity η T,S and temperature, and the correlation coefficient reached −0.998. The biological seawater viscosity η Bio was positively correlated with the number of P. globosa colonies, and the correlation coefficients is 0.747. Because the increase in phytoplankton abundance, especially during the outbreak of P. globosa and a large amount of mucus produced by the colonies could significantly increase seawater viscosity, we suggest that biological factors were the main reasons for the increase in seawater viscosity. The η Bio was completely consistent with the occurrence process of P. globosa bloom and could be used as a valuable index for P. globosa bloom monitoring.