It is a problem of universality that debris flows endangered cities and towns in the mountainous region, especially in the Hongduan Mountains.In recent twenty years，the damages in different scales take place in every year, and cause heavy casualties and losses，even destroy part of a town.The reasons that the cities and towns were endangered by debris flows are directly related to the human activity, as well as the natural factors.The cities and towns destroyed by debris flow are mainly distributed in high mountains and deep valleys or along the foot of the mountain with a great difference of height. In history, the development of of cities and towns in the mountainous region was confined by the human cognitive ability to natural disasters and the developing level of production. Most of the destroyed cities and towns were built on the fan land of debris flow ravines. Despite facing the heavy and frequent disasters,up to now the prevention of debris flow as an important problem has not been ,known and solved, Since the middle of the sixties，the disasters have been more serious because the development. of the mountains and the construction of the cities and villages is quickened,the managemnt of the construction and the plan of city and town was imperfect and the prevention to the the disasters has not been carefully thought out or not be handled properly in the construction of city. The authors made an analysis to the destruction，destructive types and danger degree of debris flow ravines according to the position of city and town on the fan of debris flows，and to the relationship of the ravines and rivers, the factors of the distribution of city and town. It is of importance for planning city and town and working out the emergency measures to prevent against debris flows.