Plasma density depletions in the equatorial ionosphere, or so‐called equatorial plasma bubbles (EPBs), are generated in the postsunset period and tend to have a very complex spatial structure. Especially, the east‐west asymmetry of EPBs has been reported by various observations. Using a high‐resolution bubble (HIRB) model, which is a newly developed three‐dimensional numerical model for the equatorial ionosphere, small‐scale structuring at the west wall of large‐scale F layer upwelling is clearly reproduced for the first time. It is not an eastward neutral wind but a vertical shear of zonal plasma drift velocity at the bottomside of the F region that plays an important role in accelerating the instability growth at the west wall and generating the east‐west asymmetry of EPBs.