Electro-optical (EO) switches with a subwavelength device length based on metal–dielectric–metal nanocavities waveguide combined with organic EO materials have been proposed and numerically investigated. The finite difference time domain (FDTD) method with perfectly matched layer absorbing boundary condition is adopted to simulate and study their properties. The FDTD simulation results reveal that these structures filled with EO materials can realize the function of switch with low-driving voltage. The wavelength conversion switch structure might become a choice for the design of integrated architectures for optical computing and communication, especially in WDM systems in the nanoscale.