Fog harvesting is one potential approach to provide supplementary water resources in arid areas. Considerable researches have been devoted to electrostatic fog harvesting technology, but there are still some problems such as high voltage, complex structure and expensive cost. Decorating micro-nano protrusions on the electrode is an effective method to lower operating voltage, improve fog harvesting efficiency, and the enhancement effect of protrusions has already been experimentally demonstrated. However, the enhancement mechanism on the microscopic level is less reported. This manuscript tries to explain why the micro-nano protrusions can enhance the discharge and fog harvesting by numerical simulation. Three key processes of corona discharge, fog droplet migration, and fog harvesting efficiency are discussed in detail, especially the influence of droplet size, tip radius of protrusion, protrusion-protrusion angle and so on. The numerical simulation results show that the inception voltage of barbed electrode decreases from 7 kV to 3 kV (decrease in 57%), and current increases significantly (e.g. 68% at 15 kV). At 15 kV, the fog harvesting efficiency of barbed electrode is higher (29.8%) than that of smooth wire (25.7%), even with less effective collection area. The collection efficiency increases with the droplet size and exists an optimized ratio (~1‰) of protrusion tip radius and wire radius to gain high collection efficiency. This research results are beneficial for understanding the microscopic mechanism of protrusions enhancing electrostatic fog harvesting and providing guidance for further fog harvesting equipment improvement.