A simple and practical modeling of the SOI (Silicon-on-Insulate) zero-gap directional coupler switch (BOA type-Bifurcation Optique Active) is proposed. The electrooptic modulating mechanism of the zero-gap directional coupler switch is investigated by using the plasma dispersion effect. The electricity character of the switch is analysed with the pn junction large injection effect. On the basis of the analyses and the theory of the single- mode optical waveguides with large cross-section, structure parameters and electricity parameters of the switch with double-mode interference mechanism are designed.