Electrochemical process is used in carbon fiber surface treatment predominantly. The cell parameters like voltage and current are usually given in average. However, because carbon fiber's resistance is quite high, the electricity potential along the fiber tow decreases rapidly, which affects not only the results but also the efficiency. Nevertheless, no literature has talked about this until now. In this paper we set up a model and got a series of formulas, apparently for the first time, capable of simulating the distributions of potential and current along a carbon fiber tow, and power consumption under different cell parameters such as fiber gauge and electrode distance. Through analysis we found that under a certain voltage between the fiber roller and the graphite cathode, the longer the fiber length soaked in the bath, the greater the attenuation of potential and current along fiber tow, and the more the useless power was consumed. It is probably considered that the distance between carbon fiber tow and graphite cathode should be as close as possible, however, when the distance was too close, the potential decayed much sharply, resulting in low efficiency. The model or the formulas can help find the optimum choice of cell parameters. (C) 2018 The Electrochemical Society.