Poly(ethylenimine) (PEI) polyelectrolytes have been widely used to tune the stability, rheology, or adhesion properties of colloidal suspensions due to their strong tendency to adsorb to solid surfaces. They have also gained importance as gene carriers in biomedical applications, in which the anionic DNA chains are complexed and condensed to form PEI/DNA polyplexes. Some reported literatures have recently shown that the overdosed PEI chains, which are free in the solution mixture, also play a vital role in promoting the gene transfection, but the reason is unclear. In this work, we present the results of using total internal reflection microscopy (TIRM) to measure the interaction forces between a Brownian colloidal sphere and a flat glass plate in the presence of overdosed free PEI cationic chains, when both surfaces were saturated adsorbed with the PEI chains. The colloidal sphere preadsorbed with PEI chains was chosen to mimic the PEI/DNA polyplex. Results for the potential energy of interaction measured for model polyplex (e.g., PEI-coated sphere) interacting with a PEI-coated glass surface in the presence of overdosed free PEI chains at various pH values and salt concentrations were presented. As can be shown by direct force measurements, the interaction potentials in NaCl salt solution are dominated by repulsive forces originating from diffuse layer overlap and gravitational attraction. However, the presence of free PEI chains in the solution mixture produces a long-ranged (>60 nm) attractive force between two PEI-coated surfaces with the range and magnitude tunable by pH value, PEI, and salt concentrations. The possible mechanisms of this long-ranged attractive force are discussed. A better understanding of this free PEI-induced attractive force will be useful in the development of improved PEI/DNA polyplexes systems for biomedical applications.