Exemplified by Chang 9-Chang 7 braided channel delta sediments of Yanchang Formation along Ruishui River outcrop profile in southwestern Ordos Basin, this paper deals mainly with sandstone reservoir and diagenseis characteristics of the Grade 3 sequence boundary. An analysis by such experimental means as thin-section identification, SEM and X-ray diffraction has revealed that the sequence boundary along the Ruishui River outcrop profile controlled the sandstone reservoir and its diagenetic process, that the main matrix below the sequence boundary is aedelforsite, while that over the boundary is aedelforsite, and that the pores are mainly secondary pores formed by matrix dissolution. The area-porosity ratio is higher over the sequence boundary than below the boundary, which is favorable for the development of the reservoir.