In this paper, we propose a simple two-dimensional (2D) analytical threshold voltage model for deep-submicrometre fully depleted SOI MOSFETs using the three-zone Green's function technique to solve the 2D Poisson equation and adopting a new concept of the average electric field to avoid iterations in solving the position of the minimum surface potential. Firstly, we obtain the 2D potential distribution in the Si-film region by using the Green's function technique to solve the 2D Poisson equation. Then, by applying Gauss's law at the Si-SiO2 interface, the initial expression of the threshold voltage is obtained. Eventually, we introduce a modified factor to compensate for the errors resulting from the charge-sharing effect in the derivations of the final threshold voltage model. The proposed model is validated against the results obtained by 2D numerical analysis and experimental data, and excellent agreements are obtained. The proposed model has an explicit expression and can be implemented into the circuit simulator.