横断山段的主体部分，主要属甘孜、阿坝、凉山三州范围。土地面积占全省一半以上，而人口数仅占全省人口总数的5.97％，每平方公里约19人，西北部仅3～5人。
本文拟就横断山北段环境对人口增加的限制作一些讨论。

The population distributes mainly in valley in North Hengduan Mountain. The environment limit to increase of population shows following.

Flat land is smaal in valley. Landslid, debris flow and eathquake distribute widely and occur frequently. Delivery of waste products of industry is difficult. Ecologe environment destroyes easily and dry valley area is incease continously. The arable land is small and the population is increase in this region, so the grain and vegetablr is not enough to meet the need of people by selfsufficiency.