Aiming at the problem that traditional carbon fiber electric field electrode is subject to the capacitive reactance effect and cannot response to low frequency signals, in this paper, a new type of carbon fiber marine electric field electrode is prepared using the modification method of concentrated nitric acid oxidation and silica sol coating. The experiment results show that large number of nitrogenoxygen functional groups are introduced to the modified carbon fiber surface, which not only increases the specific surface area and hydrophilicity of the electrode, but also reduces the effect of the capacitive reactance effect. In terms of electric field response performance, the modified electrodes can correctly respond to the LF signal with frequency as low as 1mHz. Compared with the results before modification, the response accuracy and sensitivity are improved greatly, the linearities of the modified electrodes responding to frequency of 10 mHz and 1 mHz are 33% and 21%, and the sensitivities are 0048 2 and 0050 3, respectively.