Fushan Sag is in the Beibuwan Basin is enriched in hydrocarbon. The third member of Liushagang Formation is currently an important reservoir in the northern part of the Huadong Beilian area. Its reservior lithology contains sandy conglomerate, gravel sandstone and fine sandstone，which consist mainly of lithic sandstone and feldspar lithic sandstone．In petrology，the sandstones are characterized by high composional maturity，high textural maturity and higher cement．Sandstone reservoirs from the third members of Liushagang Formation，which porosity is 8.41% 12.14% and permeability is 0.224×10-3μm2~3.472×10-3μm2，belong to the medium low porosity and ultra low permeability reservoir．Study demonstrates that mechanical compaction is main cause of tight sandstone reservoir，dissolution in medium diagenesis “A” improved reservoir property．Due to the enclosed diagenetic enviroment，feldspar and debris were not dissoluted completely. Authigenic kaolinite, although low content, can improve reservoir physics effectively，and fill pores along with illite/smectite mixed layer, illite and chlorite. Thus their effect on the reservoir is limited．Precipitation of illite/smectite mixed layer，illite，chlorite，carbonate and phase Ⅱ quautz along the fluid channel or pores are the important fators to the tightness of reservoir.