The electrochemical stability of the implantable microelectrode array is one of the most important considerations for effective neural stimulation. Electrical characteristics of a polyimide-based platinum microelectrode-electrolyte interface were presented in this paper, which could help determine some stimulus parameters in neural restoration applications. The novel 16-channel Φ-200 μm polyimide-based platinum thin-film flexible microelectrode array was micro-fabricated and an appropriate circuit model of the electrode-electrolyte interface was adopted with three different components of series capacitance Cs, series resistance Rs and Faradic resistance Rf. By using sinusoidal testing signals, the respective changing relationships between the former two components and modulus of impedance Z vs. applied current density were tested in vitro at 37.8°C. The tested results showed that the magnitude of Cs and Rs maintained respective constant values at the low stimulating current density, while with current density gradually increasing, Cs increased and Rs decreased sharply. The tested electrochemical impedance in vitro decreased with the increasing frequency.