According to the particularity of towns construction in China’s southwestern mountainous areas，this study has built constructive land suitability evaluation index system which is different to flat area. Then this study determined single evaluation factors index and comprehensive suitability index using “extreme conditions method” and “suitability index method”，and analyzed and assessed the constructive land suitability on each evaluation unit polygons and its suitability level by using GIS software. Taking Mangshi of Yunnan Province as an example to evaluate land suitability in mangshi’s main cities and towns around the gentle slope，the results show that，in the evaluation area，the area of land suitable for construction and no- suitable for construction respectively account 81.11% and 18.89%. And first-class，second and third-class of land suitable for construction should be composited the ratio of about 12：28：60. The suitability evaluation index system and method developed in this paper is applicable for evaluating the construction land suitability in mountainous areas of southwest-China，and will provide theoretical and technical support for land policy measures which strengthen the protection of farmland and promote the scientific development of urbanization.