Junctionless double-gate transistors which have obvious advantages over traditional junction transistors are becoming a hot topic in the field of nano Silicon-On-Insulator(SOI) devices nowadays. Based on 2-D Poisson equation, an analytical model is derived to calculate potential distribution in the channel. Based on this model, threshold voltage of the junctionless double-gate transistors can be obtained. By using this model and device simulator MEDICI, effects of gate voltage and parameters of device structure on potential distribution and threshold voltage are investigated in detail. This model is simple and has a good match with the simulation results.