Water-sensitivity damage is a significant problem for production of ultra-low permeability formation located in Yanchang oilfield, China. It is important to understand the water-sensitivity damage accurately and enhance the formation production. In this paper, we quantitatively evaluate the all water sensitivity damage on porous media, and obtain their negative influence to the production. Core flow experiments are designed to evaluate the water sensitivity damage degree. NMR(Nuclear Magnetic Resonance) method is introduced to compare images before and after the water fluids displacement through rock cores, and calculate the degree of permeability and porosity damage. QEMSCAN(Quantitative Evaluation of Minerals by Scanning electron microscope) technique is used to obtain the damage mechanisms by compare changes of pore diameter, mineral contents and distribution forms at inlet and outlet sides before and after water sensitivity damage. Numerical simulation method of ultra-low permeability is utilized to calculate the effect of water sensitivity on production. The results indicate that the clay swelling and migration are the main cause of water sensitivity for ultra-low permeability. That causes gradual increase of bound fluids and gradual porosity decreases from inlet to outlet. Porosity decreases by 1.61% on average. In the distribution of pore diameter, the pore size larger than 35μμm decreases and the pore size smaller than 35μμm micron increases. Further, result of QEMSCAN points out that Illite/montmorillonite and Chlorite/montmorillonite mixed-layer minerals bring out clay swelling, and illite and chlorite cause clay migration. At last, the result of numerical simulation method of ultra-low permeability shows that water sensitivity damage can result in reducing of oil production, decreasing of injection volume of water well, the smaller swept area of water injection, the lower displacement efficiency, the higher the pressure gradient, the greater pressure loss at the injection end, and the more difficult to inject and produce. From micro-mechanism to macro-production, our research fully and clearly reveals the damage of water sensitivity to ultra-low permeability reservoirs. Our research works have been successfully applied to ultra-low permeability reservoirs of Yanchang oilfield to reducing formation damage during development.