Abstract: The N-sandstone, of which with the characters of poor diagenetic grade, liable to weathering and disintegration, is the extremely soft rock. The N-sandstone is widely distributed in Lanzhou Basin, Gansu Province, China. The structure of the huge thickness N-sandstone will be easily disturbed under the effect of ground water and then softening. That would lead to the instability happened in the engineering basement, especially in the tunnel structure. For studying the engineering geological condition of N-sandstone, the investigation and test data of the Lanzhou to Chongqing Railway and Lanzhou to Zhongchuan Railway was studied and then obtained the physico-mechanical properties of the sandstone. Additionally, through applying scanning electron microscope technology the microstructure change character under the effect of different water content was studied also. Finally, the main effect factors of the engineering geological condition were determined and as follows: water content, clay content, anion concentration of groundwater and porosity. The catastrophe progression method was adopted to establish the engineering geological condition quantitative evaluation model of sandstone. The engineering geological condition of N-sandstone can be divided into five grades: severe(S0.64),bad(0.64 S0.71),general(0.71 S0.79),well(0.79 S0.87) and perfect(S0.87). The evaluation result about the Taoshuping Tunnel of Lanzhou-Chongqing Railway was between 0.57 to 0.77. The engineering geological condition was sever and general. The evaluation result was in keeping with the actual situation.