This paper aims to reveal the water transport behavior and the degradation mechanism of the retardation performance of the polyimide materials used in the encapsulation of implantable electronic devices (IEDs), and provide a method for seeking the evaluation methods of the retardation performance and lifetime of the packaging materials, and to improve the protection ability of IEDs basic experimental data and theoretical references. Select simulated body fluid as the research medium, and use weighing experiment, electrochemical impedance spectroscopy (EIS), morphology characterization and porosity measurement and other technical means to study the water transmission behavior in the polyimide coating and the coating retardation performance. At the same time, the influence of biological macromolecules on the water transport behavior in the coating is discussed. The results show that water transport behavior includes four stages:in the first stage, water absorption rate increases rapidly in a short time, which means the coating surface is wet by water. In the second stage, the transport process of water conforms to Fick’s diffusion. The diffusion coefficient of water at this stage is only 5.6×10–15 cm/s. In the third stage, the water absorption rate increases suddenly, which is because that water molecules interact with the polymer to form bound water. The coating has reached the saturation state in the fourth stage. Water exists in two states:free water and bound water. The deterioration of the coating retardation performance is due to the low adhesion of the coating on the substrate, which is only 1.78 MPa. When corrosive particles such as water enter the coating, the coating will foam and the retardation performance will deteriorate. Besides, when water and other corrosive particles enter the coating, it will cause the coating to foam and degrade the retardation performance. As a result, we can draw the conclusion that the water transport in the polyimide coating lowly, and the macromolecules in the body fluid will further prevent the penetration of water. Therefore, polyimide is an ideal internal encapsulation material, and the service life of the coating can be increased by improving the bonding force between the coating and the substrate.