Carbon fiber electric field electrodes have the advantages of low cost, good chemical stability, and tunable performance, which can be used to develop high-performance ocean electric field sensors. In this paper, surface modification of polyacrylonitrile based carbon fiber was carried out by in-situ electrochemical polymerization of aniline by using ammonium bicarbonate weakly basic solution as electrolyte to fabricate marine composite electrode, and its electrochemical and electric field response performance were studied respectively. The results showed that reduced polyaniline formed on the surface of carbon fiber, in which it appeared characteristic redox peaks. The AC impedance spectrum showed that the modified electrode surface equivalent circuit increased the film capacitance and film resistance, and the electric field response changed from pure capacitance coupling to a hybrid mechanism of capacitance and resistance. It was beneficial to the response of low frequency and low intensity signals. In the electric field performance test, the electric potential drift of the modified electrode pair was as low as 0.66 mV/d, and it induced well to low frequency and low intensity electric field signal at 10 mV, 10 mHz, in which its the minimum linearity was 0.072%. The mass loss of polyaniline film after the scour of water was 6.30%, compared with the polyaniline film electropolymerized in hydrochloric acid solution (mass loss 7.55%), it may have better binding force. So the polyaniline modified carbon fiber composite electrode would be utilized to develop a new generation of low-cost, high-performance electric field sensors.