Abstract: Surrounding rock structures of steep seams and gently inclined seams are different. Basic roof and immediate roof of horizontal section top coal caving were beside the working face in steep seams. A judgment criterion of roof destruction was attained by analyzing tension to basic roof. According to the judgment criterion, it was verified that only one part of overlying strata was loaded to exposed rock board. There is unloaded arch structure in overlying strata. The unloaded arch structure bears most weigh of overlying strata and effects later strata behavior of the working face. It is pointed out that the unloaded arch structure is a dynamic expansion structure. It will move towards roof as original balance of arch was broken and keeps a new balance. The scope of arch is a process constantly to expand from balance to instability.