In this paper, we report the design, preparation and properties of epoxy-mercaptan polymer/nematic liquid crystal (LC) composites with electrically switchable optical transmittance. It is found that the polymer microstructure can be regulated from porous structure to microspheres or beads by appropriately modulating the curing temperature, the molecular structure of mercaptan, as well as the compositions of epoxy, mercaptan and nematic LC host. Under an external electric field, the thin films based on such composites can switch from opaque to transparent. The electro-optical performance is found to be highly dependent on the polymer microstructures and the optimum characteristics can be achieved in the composite with a microstructure of smaller microspheres. The possible mechanism for the formation of polymer microspheres and the effects of the microstructures on the electro-optical characteristics of the composites are also discussed. This work is expected to provide an efficient way to control the microstructure of epoxy/mercaptan polymer and develop the composites with switchable optical transmittances for both civil and military applications.