The interaction between sodium dodecyl sulfate (SDS) and the cationic polymer poly(diallyldimethylammonium chloride) (PDAC) was investigated by viscosity, conductivity, and dynamic light scattering measurements at 298 K in order to monitor the changes in the charged nature and size of SDS + PDAC complexes. The experimental results showed that the critical aggregation concentration (CAC) value of SDS + 1 % mass fraction PDAC is almost 2 orders of magnitude lower than the critical micelle concentration of SDS. The viscosity of the SDS + PDAC solution increased with decreasing shear rate, exhibiting non-Newtonian behavior. The minimum viscosity and hydrodynamic diameter of SDS + PDAC complexes at the binding site confirmed the contraction of the polymer chain and the formation of a more compact structure. When the SDS concentration was above the CAC, the viscosity and hydrodynamic diameter increased, indicating that the PDAC chains first extended and finally collapsed and precipitated. The binding degree of SDS + 1 % mass fraction PDAC was from beta < 1 to beta approximate to 1. The interaction between SDS and PDAC can be divided into different characteristic SDS concentration ranges. At low SDS concentration, the surfactant-polymer system is a thermodynamically stable solution of the surfactant + polymer complex molecules. Above this critical concentration, the system is an unstable colloid dispersion of SDS + PDAC complex particles. The interaction between SDS and PDAC is favored and strong.