Quantum effects play an important role in determining the Double-Gate (DG) MOSFETs characteristics. The Non-Equilibrium Green's Function (NEGF) formalism provides a rigorous description of quantum transport in nanoscale devices. The traditional NEGF is heavy in computations and not suitable for 3D or even 2D device simulation. In this article, we propose a method that reduces the simulation time dramatically without loss of accuracy. The proposed method is used to simulate a 5 nm channel length DG MOSFET. The simulation time is reduced from 72 minutes to 11 minutes per bias point on a home PC: Intel&reg Pentium 4 CPU 2.4GHz, 768 MB RAM.