Sand blocking fences at the front of the railway protective system in Shapotou section of Baotou-lanzhou Railway are not only the important barriers that obstruct blown sand flow, but also the prime conditions for the  formation and development of the special dunes that we defined as fence dunes in this paper. Observations on dune morphology and air flow field indicate that windward slope of the fence dunes is steeper than that of the natural dunes but leeward slope is gentler than that of the natural dunes. The ratio of the horizontal length of leeward slope to windward slope of the fence dunes is about 0.7, much larger than that of the natural dunes (0.3). Sand blocking fences influence the near surface air flow field significantly. However, if the fences are missing or buried by the accumulated sands, the structure of air flow field over the fence dunes will be similar to that over the natural dunes. Changes of shear wind velocities (u*) along the windward slope of these two types of dunes exhibit a similar tendency that increases from the windward slope toe to the middle-upper slope of the dune and then decreases to the dune top. The only difference is that both increase extent of u* from the windward slope toe to the the middle-upper slope of the dune and decrease extent of u* from the middle-upper slope to the dune top on the fence dunes are larger than those on the natural dunes. This difference is significant in development and growth of the fence dunes.