Sayi Reservoir is one of the most important oil layers in the northern Saertu oil field. For a long time, Sayi Reservoir used to be considered to belong to external delta front microfacies with the front sheet sand well developed, but the underwater distributary channel has not been found. With a further exploration and development and well pattern infilling, the distinct underwater distributary channels are found so it is established that Sayi Reservoir belongs to inner delta front microfacies, which include the underwater distributary channel, the front sheet sand, the inner front sheet sand and the external delta front microfacies. Because the underwater distributary channel is an important mark for identifying external and inner delta front microfacies, the typical underwater distributary channel is recognized from the sedimentary facies maps of Sayi Reservoir through analyzing deposition character and establishing logging facies models. The underwater distributary channel sand and the front sheet sand are distinct in sand shape, scale, distribution area, continuous condition and physical property, which decide the development plan and effect. This study can offer not only a new viewpoint for the remaining oil evaluation but also an important geologic evidence for Sayi Reservoir.