Electrical Resistance Tomography (ERT) has ever-increasingly been used in industrial detection process. In the ERT sensor of dredging engineering, the measuring electrodes is equipped in the insulation layer whose length plays an important role in the ERT measurements. But the effect of the length fails to be studied so far, and usually is determined by experiences. In this paper, a finite element numerical model is constructed to simulate and test the ERT system in dredging engineering, analyzing the influence of the length. Considering the limited maximal length of insulation layer and measurement quality, the accepted length is obtained. Experiments validate the proposed method when computing the solid-phase fraction in practice.