Electroactive polymers (EAPs) are promising materials for actuation and energy harvesting applications. Among the EAPs, polyurethane (PU) material is of considerable interest given its high values of deformation under an electric field. The electromechanical properties were found to be dependent on the processing technique and the thickness of the film. To understand this relationship, a comprehensive study was carried out on polyether-based thermoplastic PU elastomer films elaborated by solution casting with thicknesses between 12 and 220 μm. Microstructural, dielectric, mechanical, and electrostriction studies were conducted. Thin films present a lower strain for a given electric field compared to thick films. The films exhibit a structural gradient along the thickness direction: a fast evaporation in the upper part of the film close to the interface with air inhibits the phase separation but a more favored one in the lower part. This is consistent with the modeling based on the gradient of dielectric constant and the experimental, mechanical, and dielectric characterizations.