The inner architecture characterization of reservoir is the base and key factor to improve the waterflooding effectiveness and enhance the oil recovery. Taking the Es281 sand unit of Shengtuo oilfield in Dongying depression as an example, which is a typical underwater distributary channel of delta front reservoir, the inner architecture of the composite sand body was characterized by close spacing architectural analysis. The study shows that the depositional interfaces in the underwater distributary channel can be classified into 5 levels. The fifth level interface restricts the composite underwater distributary channel, which is the stable mudstone with major thickness and near horizontal attitude. The fourth level interface restricts the single channel, which is mainly the muddy intercalation or lithological discontinuity surface with near horizontal attitude and continuous distribution in same single channel, but with discontinuous distribution among the different single channels. The third level interface restricts the accretion sand body in the single channel, and the upper and lower lithofacies of the third level interface is accordant. The distribution range of the third level is confined by single channel and the attitude is also near horizontal. The second and first level interfaces restrict the bedding system and cross bedding with core scale. The composite underwater distributary channel sand body is overrode vertically and built up laterally by single channel. The lateral border of the single channel can be identified by inter-distributary bay, channel marginal deposit, top surface altitude difference of sandbody and scale difference of sand body. The most important inner architecture characteristic of the underwater distributary channel is the contemporaneous deposit of single channel but independent sand body.