We investigate the thermodynamic properties of a water-soluble polymer poly(ethenol) (PVOH) with and without ionic surfactant sodium dodecyl sulfate (SDS) by viscosity, surface tension, and dynamic light scattering (DLS) measurements at 298 K. The viscosity, refractive index, and hydrodynamic radius values of polymer PVOH aqueous solutions without surfactant SDS increase with increasing PVOH concentration, but the surface tension of PVOH aqueous solution decreases with increasing PVOH concentration. For SDS+PVOH aqueous solutions, both viscosity and surface tension values decrease with an increase of SDS concentration when SDS concentration is lower than the critical aggregation concentration (CAC) of SDS+PVOH aqueous solutions, and the relative minimum viscosity value indicated the shrinkage of the polymer PVOH chain resulting in the formation of compact structure at the binding site of SDS+PVOH. But the viscosity increases with an increase of SDS concentration when surfactant SDS concentration is larger than CAC value mainly due to the extension of PVOH chains. We obtain the CAC value of SDS+PVOH aqueous solutions from the corresponding SDS concentration at the first minimum value of both surface tension and hydrodynamic diameter. The CAC value is much smaller than the critical micelle concentration (CMC) of SDS which indicated the strong interaction strength between ionic surfactant SDS and water-soluble polymer PVOH due to the large negative value of Gibbs free energy change.