[Objective] Fine root is the primary organ for trees to absorb nutrients and water, and plays a significant role in the nutrient cycling and energy flowing of terrestrial ecosystems. Despite our understanding of the importance of fine roots for carbon and nutrient cycling, lack of understanding of acclimation and adaptation mechanisms of fine roots under different environmental conditions is a key shortcoming in the future projection about the consequences of climate change. The purpose of this paper is to compare the plasticity of fine root morphology under different environmental conditions and the morphology of fine roots of two tree species, and to analyze the effects of environmental factors and ectomycorrhizal colonization on fine root morphology. [Method] This paper takes the dominant species of Larix gmelinii and Betula platyphylla in the permafrost regions of the Xing 'an Mountains (Yichun permafrost degraded area, Nanwenghe permafrost degraded sensitive area, Mohe permafrost area of northeastern China) as the research object, the intact root segments were sampled by excavation method in the growing season. We measured the diameter, specific root length, specific surface area, tissue density and ectomycorrhizal colonization rate of primary development roots (1st and 2nd root). [Result] Soil available nitrogen, soil total carbon and monthly mean temperature of growing season were the main factors affecting the morphology. The fine root morphology of Larix gmelinii differed little (P > 0.05) among the three research sites, and the ectomycorrhizal colonization rate made significant difference (P < 0.05). For Betula platyphylla, the diameter, specific root length, specific surface area and ectomycorrhizal colonization rate were significantly different (P < 0.05), but tissue density did not differ (P > 0.05). The diameter of fine root of Betula platyphylla was the smallest in the Nanwenghe permafrost degraded sensitive area, but specific root length and specific surface area were the largest; the morphological characteristics of fine roots and the colonization rate of ectomycorrhizal fungi exhibited different between two tree species; and the colonization rates of Larix gmelinii and Betula platyphylla were significantly positively correlated with fine root diameter (r = 0.64, P < 0.01; r = 0.61, P < 0.01). [Conclusion] The soil nutrient and the monthly mean temperature of the growing season are the main factors influencing the morphology of fine roots. Larix gmelinii mainly relies on ectomycorrhizal fungi to adapt to environmental changes, while Betula platyphylla is adjusted by fine root diameter, specific root length and specific surface area and ectomycorrhizal infection rate in response to environmental spatial heterogeneity and the existence of ectomycorrhizal fungi is a crucial alternative absorption strategy.