Abstract: The crude oil charging degree is high in the Chang 7 tight oil reservoir of Yanchang formation in Ordos basin, the average oil saturation is more than 70%, and ultra-low water saturation is observed in the reservoir locally. Previous researchers seldom considered the influence of the back pressure at the outlet end of the core on oil and water flowing behavior when they carried out crude oil charging experiments on tight cores. During the displacement, only the fluid around the inlet formed the pore pressure under displacement pressure to overcome flow resistance and the pressure at the outlet end was atmospheric pressure, resulting in significant differences between the water saturation obtained from the displacement and the water saturation gained from logging analysis. In order to study the influences of back pressure on ultra-low water saturation forming process during crude oil charging in tight oil reservoirs, the core from Chang 7 tight reservoir of Yanchang formation is selected to carry out a single-phase flowing simulation experiment under different pore pressures with added back pressure at the outlet end, and crude oil charging experiment is performed based on the selected back pressure. The experiment result shows that with the added back pressure the required displacement pressure gradient significantly decreases during the single phase flow, whereas, with the same low pressure gradient a larger flowing rate can be measured. During two phase displacement, the obtained water saturation with the added back pressure is consistent with the result interpreted by logging data, indicating that the forming process of ultra-low water saturation can be represented under the added back pressure. It is considered that an added back pressure is of great significance to improve the test efficiency of liquid flowing experiment for tight cores and to simulate the ultra-low water saturation forming process during crude oil charging