To probe the interactions between flexible and rigid macromolecules in polymer flooding systems，the rheological properties of mixed solutions containing flexible partially hydrolyzed polyacrylamide （HPAM） and a rigid anionic exopolysaccharide（EPS）secreted by a deep-sea mesophilic bacterium Wangia profunda SM-A87 were studied in detail. Results showed that mixing of HPAM and EPS leaded to a viscosity decrease of their mixed solutions in the semidilute regime，which was rarely seen in hydrogen-bonded complex systems. With increasing electrolyte concentrations and shear rates，the viscosity decrease of HPAM/EPS solutions could turn into the viscosity increase. The viscosity increase of HPAM/EPS solutions was also observed when tuning the solution pH to be ＜5 or ＞11. The viscosity decrease or increase of the HPAM/EPS solutions resulted from variation of the molecular configuration that could be influenced by electrolyte concentrations，pH，and shear rates. Compared with the HPAM system，the HPAM/EPS complex system exhibited stronger salt resistance and thus great potential for application in tertiary oil recovery. This work can direct research on other similar systems composed of flexible and rigid anionic macromolecules.