The invention relates to a mountain torrent disaster risk evaluation method based on a polymorphic system theory. The method includes the following steps: collecting data; carrying out mountain torrent disaster status survey; grading influence factors; establishing a model; and determining different risk grades. Through collection and arranging of basic information of hydrology and landform with a small basin being a unit, technological means such as GIS, satellite remote sensing and hydrological analytical calculation is fully utilized to extract related data, combined with mountain torrent disaster field survey data, a small-basin rainstorm flood analysis is made, a mountain torrent-threatening residential area mountain torrent disaster status defense capability is evaluated, and village danger zones along a river are scientifically divided, through analysis evaluation, basic characteristics of small-basin rainstorm flood of a mountain torrent disaster danger zone and a personnel and property distribution situation are mastered relatively comprehensively and accurately, a relation among a rainstorm, mountain torrents and a disaster is analyzed, and critical rainfall of villages, market towns and towns along the river that may cause occurrence of a mountain torrent disaster is analyzed, thereby providing support for further improving a mountain torrent disaster prevention and control capability and sustainable development of social economy.