The basic and supporting functions of critical ecological space in the ecosystem are particularly important. It is of great significance for homeland security and regional sustainable development to identify and evaluate the scope of the critical ecological space. Taking the economic belt of Tianshan northslope as the research area, we constructed a critical ecological spatial evaluation system suitable for the fragile and sensitive environment in the northwest cold and dry area. With the help of GIS and RS, the spatial scope of the critical ecological space was identified and graded from the importance and sensitivity of the ecosystem service function, then analyzed the conflict of land use ecological security. The results showed that: (1) The critical ecological space generally consists of the bottom line type and crisis type ecological space with area 45318 km², accounting for 53% of the total area, mainly distributed in mountainous, desert and the northwest of the agricultural reclamation area. (2) The ecosystem service function is significantly related to vegetation. The medium important, important and extremely important areas account for about 20.9%, which are concentrated in the southern piedmont grassland and the central agricultural reclamation region. The ecosystem service function are weak as a whole, and the dominant factors are water conservation and biodiversity conservation function. (3) The primary sensitivity level of the study area is medium sensitive. The sensitive and extremely sensitive areas account for about 37.7%, which are distributed in high mountain area, gobi desert terrain and the transition area between low mountain and oasis, principally reflected in desertification and salinization. (4) The conflict level of land use ecological security in this region is relatively high, there are urban construction land 45% and cultivated land 56.5%occupy the critical ecological space. This study can provide reference for the optimization of regional land spatial distribution and ecological pattern, and play a complementary and referential role for the relevant theoretical and empirical research.