Lancang-Mekong River is a socially, economically and environmentally important trans-boundary river for China and other five countries in the SouthEast Asia. Based on the realities of the river basin, the paper proposes to construct transboundary environmental impact assessment (TEIA) mechanisms under the international legal framework of sustainable development and suggests that such cooperative mechanisms as ASEAN-China Dialogue Relations, ASEAN-China Free Trade Areas and Mekong River Commission between China and ASEAN countries lay the cooperative foundations for developing TEIA in the river basin. Currently, the feasible aim of TEIA in the river basin is to make the riparian countries accept the ideas of TEIA, develop TEIA mechanisms in a procedural sense and promote a cooperative atmosphere in good faith. In particular, the paper finally discusses the two key issues of developing procedural TEIA in the river basin, i.e., accessibility to relevant environmental information and promotion of public participation in decision-makings.