第二语言习得是现代应用语言学的核心研究领域，旨在揭示人类习得非母语的心理机制、认知规律与环境影响因素。20世纪中后期以来，以克拉申为代表的研究者提出的第二语言习得理论体系，尤其是输入假说与情感过滤假说，对全球范围内的语言教学实践产生了深远影响。本文以第二语言习得核心理论为依托，重点梳理可理解输入、情感因素、学习动机、课堂环境等对习得效率的作用机制，结合国内相关实证研究成果，分析当前我国外语教学中存在的输入不足、情感过滤偏高、输出机会有限等现实问题，并从优化输入材料、降低学习焦虑、强化互动输出、注重个体差异等方面提出提升习得效率的可行路径。研究认为，可理解输入是习得发生的前提条件，积极的情感状态是习得顺利推进的重要保障，而输出与反馈则是实现语言内化的关键环节。只有实现输入、情感与输出三者的协同作用，才能真正提高第二语言习得的效果与效率。

Second language acquisition is a core research area in modern applied linguistics, aiming to reveal the psychological mechanisms, cognitive patterns, and environmental factors involved in humans acquiring a non-native language. Since the mid to late 20th century, researchers represented by Krashen have proposed theoretical frameworks for second language acquisition, particularly the Input Hypothesis and the Affective Filter Hypothesis, which have had a profound impact on language teaching practices worldwide. This article, based on the core theories of second language acquisition, focuses on reviewing the mechanisms through which comprehensible input, affective factors, learning motivation, and classroom environment influence acquisition efficiency. Drawing on relevant empirical studies conducted domestically, it analyzes current issues in foreign language teaching in China, such as insufficient input, high affective filters, and limited output opportunities, and proposes feasible approaches to improve acquisition efficiency, including optimizing input materials, reducing learning anxiety, strengthening interactive output, and emphasizing individual differences. The study suggests that comprehensible input is a prerequisite for acquisition to occur, a positive affective state is an important guarantee for smooth acquisition, and output and feedback are key processes for language internalization. Only by achieving a synergistic effect among input, affect, and output can the effectiveness and efficiency of second language acquisition be truly enhanced.