弛豫时间近似是固体物理学中讨论输运问题的重要理论方法，也是凝聚态物理研究中常用的理论工具．很多固体物理学教材中缺乏对弛豫时间近似物理思想的介绍，也未能突出应用弛豫时间近似的优势．本文讨论了弛豫时间近似的物理基础，通过概念性分析得到数学表达形式，并严格求解出弛豫时间近似下非平衡分布函数的形式解，建立一套形式简洁、逻辑清晰的教学内容体系，有助于学生全面深入地理解弛豫时间近似并能够将其应用到凝聚态物理研究．

The relaxation time approximation is an important formalism to deal with the transport phenomena in solid state physics. It is also a frequently applied method in the research of condensed matter physics. The physical background and advantages of the relaxation time approximation is usually missing in many solid-state physics textbooks. Here the mathematical expression is obtained by analyzing the physical process of electron collisions during transport. Then the formal solution of the non-equilibrium distribution function under relaxation time approximation is analytically solved. In this paper concise and logical contents for teaching the relaxation time approxi mation in solid state physics are established，which are helpful for students to fully and deeply understand the related physics and to apply the approximation in the research of condensed matter physics.