Abstract: Based on the outcrop observing,core describing and data analyzing of well logging and testing,as well as labor measurement,the sedimentary systems of Upper Triassic Yanchang Formation in Ansai district is researched involving systems' classification,characteristic and evolution.The result shows that the lake and delta depositions are the main sedimentary systems in study area.The former is made of shallow lake and deep lake while the later are made of delta plain,delta front and mat sands.During Chang8 period,in the study area developed the Zichang-Wuqi delta and Yanan-Ganquan delta.In Chang7 period,Ansai delta began to form,and in Chang6 the delta get to its highest development.The delta fronts connect each other and possess the largest area.In Chang4+5 period shallow lake is the main subfacies.The delta front is limited.During Chang3 and Chang2 periods,the delta plain is dominantly developing.The evolution of the sedimentary systems formed three reservoirs and cap assemblages.As a result the intervals of Chang8,Chang6 and Chang2 are main exploration targets.