Understanding the conformational dynamics of polymer chains interacting with colloidal particles is of great significance to manipulate the properties of nanocomposites and polymer-solid interfaces. Herein, the complexation between high molecular weight poly(ethylene oxide) (PEO) and silica nanoparticles (SPs) in dilute solution was investigated with viscometry, electron microscopy, and scattering techniques. In the presence of SPs, PEOs rapidly collapsed from an extended coil to a compact conformation onto the surface of silica, which simultaneously caused a decrease in solution viscosity. A viscosity minimum together with a compact complex structure was achieved. Further addition of SPs induced polymer chains to extend into the coil state, leading to a substantial viscosity increase and structural expansion of the complex. This phenomenon was rationalized by the delicate balance between noncovalent forces including hydrogen bonding between polymer and SPs, as well as electrostatic repulsion between SPs. A series of physiochemical conditions such as pH, ionic strength, polymer molecular weight, and particle size were systematically explored to unravel the factors governing polymer conformation changes. We envision that this research will shed light on the fundamental understanding of polymer-colloid nanocomposites at the nanoscale level and provide guidelines for the design of functional composite materials.