High-quality bulk GaN substrate has led to the demonstration of vertical GaN power devices. Aiming at realizing effective edge termination and avoiding premature breakdown, we propose a new fluorine-ion-based junction termination extension (F-JTE) method, which is well suited for vertical GaN devices in which the selective p-type region is difficult to form. The influences of the F-JTE and its key parameters (e.g., dose, thickness and width) on the electric field distribution and breakdown voltage have been extensively investigated and revealed by TCAD simulations. The negatively charged F-JTE region can effectively spread the electric field away from the main junction, leading to increased breakdown voltage. With a partial recess step, a double-zone JTE with a graded distribution of charges can be formed, which can further improve the breakdown voltage and process latitude.