Due to lack of sufficient data previous research mostly studied the petroleum geology character with limited 2D seismic data and logging data, so, the understanding is limited. On the basis of former understanding, and combined the 3D seismic data with core,well logging and logging data of new wells, we deeply studied the sedimentary facies and rules of distribution of down-Cretaceous in Zuunbyan sub-basin 3D work area, the author thought that, in the period of down-Cretaceous deposition, under the control of adjacent structure, the lake basin water body of study area went through shallow to deep to shallow, and relevantly formed phase sequence from stream delta front，shallow to semi-deep and deep lacustrine to delta front. Source of favorable sediments are mainly from west hog of Zuunbyan sub-basin.