Based on the symbiotic mechanisms of frozen\|ground and marshes, using large-scale mapping, drilling for soil strata and plant investigations, the effect of broad valley landform on the formation of marshes in the northern Da Hinggan Mountains was studied. It is found that the broad valley has a cube-type structure in landform, and has several important eco-environmental functions. Surface water, including meltwater from snow and ice and thawing water from frozen soil, converges into the valley bottom from both sides, forming a multi-sources water supply to the marshes. Cold environment is characterized by high latitude, high elevation, frequently freezing-thawing on the ground surface and in the near surface layers, as a result, extensive presence of frozen soil and permafrost. This cold and moist environment is conducive to the development of marshes, which can generally be classified into four vegetation types: bush, grass, moss and forest marshes. However, in the middle part of the broad valley,Larix genelini fails the constitutive role in the communities owing to its degradation; the marshes do no belong to forest marshes in the Da Xinggan Mountains as classified in the past. Instead, the marshes generally belong to the shrub-, meadow- and moss-dominated ones. Climate analyses show that the study regions should belong to subfrigid zone. The marsh flora has some characters similar to that in alpine-arctic regions. The occurrence of tundra marshes, seasonally frost mounds and palsas are rather localized. Overall, the marshes in the broad valley display the features of those under sub\|arctic climate and in environment.