Electrostatic precipitators make use of corona discharge phenomena to remove airborne dust particles. Exact assessment of the electric field and charge density distribution is essential to understand the particle behavior and the flow dynamics inside the electrostatic precipitators. The Poisson and charge conservation equations were solved to evaluate the electric field and charge density distributions in the negative wire-to-duct electrostatic precipitator. In this article, a novel computation method calculating the plasma region thickness was presented with the plasma region model. Instead of the conventional Kaptzov’s hypothesis, a boundary condition for the charge density was suggested as a function of applied voltage. When the computation model and the charge boundary condition above were applied to previous experiments, the results showed good agreements with the experimental data. The estimated plasma region thickness was approximately 1.5–2.5 times greater than the wire radius in the wire radius range of 0.15 mm to 1.6 mm.