Abstract: The dielectric constant of wave transmitting components is the key parameter to characterize its electromagnetic radiation characteristics, which is of great significance for the stable operation of modern antenna system. Free space method is widely used in the measurement of dielectric constant because of its non-contact and non-destructive characteristics. However, the traditional free space method usually requires the tested part to be a flat plate structure with smooth surface. The reason is that the electromagnetic wave radiated by the horn antenna will have multiple reflections and refractions on the surface of the tested part. If the tested part has curvature, it will cause the surface scattering phenomenon that is difficult to estimate, which will affect the final measured transmission parameter S₂₁ and further interfere with the dielectric constant extracted from S₂₁. Therefore, this paper proposes an error calibration method to correct the influence on S₂₁ caused by many factors, such as surface scattering, which are difficult to be solved by theoretical analysis, when measuring complex surface wave transmitting components such as radome. This method is verified by electromagnetic simulation software, and further combined with the actual measurement of arc plate, its effectiveness is verified. The results show that this method has good feasibility, and is helpful to reduce the constraints of the traditional free space method on the geometric shape of materials.