Electrical field (EF) is a popular tool for both basic research and clinical applications, its actions on neuronal activities have been investigated from physiological mechanism and dynamics. However, few studies explore its modulatory influence on neuronal computation from the point of view of dendritic sublinear integration caused by passive dendrites which play an important role in neuronal computation. Here with a reduced biophysical model this problem is explained by observing the impact of EF on neuronal computation and dendritic sublinear operation. It is found that the positive EF results in more linear dendritic sublinear integration because of hyperpolarization in distal dendrites together resulting in higher neuronal excitability in neuronal computation but negative EF inhibits this ability due to more pronounced dendritic sublinear operation resulting from the hyperpolarization in distal dendrites. Further, we explain the modulation of positive EF on dysfunctional neuron combining with Feature Binding Problem. This work builds the gap between neuronal computation and dendritic sublinear operation, which is helpful to understand the modulation of EFs on brain functions.