Biochar is a kind of carbon material derived from biomass through pyrolysis under anaerobic or anoxic conditions. Because of its merits of storing carbon, fixing soil pollutants and recycling wastes, it is more and more extensively used in agriculture and environment. Its stability in the environment determines stability of its environmental effects and hence an important index in evaluating environmental functions and significance of biochar. In this paper, emphasis was mainly laid on elaboration of potential physical, chemical and biological decomposition processes biochar might undergo in the environment and their influencing factors, and analysis of relationships between properties of the biochar per se and stability of the biochar in the environment. The paper pointed out that physical fragmentation, physical migration, chemical dissolution, chemical oxidation, biological degradation, and properties of the biochar per se were the main factors that jointly affected stability of the biochar in the environment. In the end, the paper brought forth some topics that are worth noticing for future research in this field, that is, (1) release and transport behaviors of biochar micro-particles under the action of water flow and their affecting factors; (2) relationship between redox activity and chemical oxidation of biochar in the environment; and (3) physical, chemical and microbial decompositions of biochar in plant rhizosphere. A systematic and in-depth study on roles of biochar in the environment is contributive to improvement of the knowledge about processes of biochar loss and full understanding of stability of biochar in the environment.