The slotwaveguide is widely used in the integrated optical waveguide devices and systems. Therefore it is very important to develop a practical theoretical method and model for guiding the design of such devices and systems. By means of analyzing the physical mechanism of the slotwaveguide and the modes transmitted in it, a novel theoretical model was built to describe the electric field distribution in the slotwaveguide. The novel theoretical model was developed based on the conditions of regarding the nanowaveguides and the slot region as a whole system and of introducing both the guided wave and the radiating wave in solving the Helmholtz equation. In order to verify the proposed theoretical model, the theoretical results were compared with RSoft simulation results, a practical optimum design reported by other researchers, and a theoretical method developed by other researchers. The comparison results show that the proposed theoretical model has higher accuracy in describing the slotwaveguides. Finally, the significant condition of the proposed theoretical model was analyzed and determined. Take a typical SOI slotwaveguide with the nanowaveguide size of 180 nm×300 nm as an example, the critical slot width worked out at 96.2 nm.