换元法是指解决数学问题时，根据问题的特征或变量关系引进新的辅助元，替换原问题中的数字， 变量或代数式等，使得原问题得到解决的方法。换元法是中学数学教学及练习中的重要思想方法，贯穿于中学数学全程。其基本形式有:常值换元，线性换元，整体换元，分式换元，三角函数换元等。 数学竞赛旨在鼓励学生思维创新，从近年来的国内外知名数学竞赛命题方向可以看出，数学竞赛题正由早期的重视数学计算能力及变量关系的偶然性，向重视思维能力及挖掘规律的普遍性转换。 鉴于此，本文以近期的数学竞赛题目为基础，以换元法的应用形式特点及其对于解题思维的作用作为切入点，在对换元法的各种应用形式作一分类综述后，选取综合应用换元法的例题说明其解题的切入点与应用过程。通过综合分析，给出应用换元法的题目分析和解题建议。

Substitution method means to solve problems in the way of creating new parameters to replace the numbers, parameters and algebraic expression in the problem according to the features of the relationships of the original parameters. The method of substitution has been proved useful and important throughout the middle school mathematics. The main forms of the method include substitution to values, substitution to linear combination, substitution to entirety, substitution to fraction, substitution to triangle functions and etc. The aim of middle school mathematical competition is to encourage teenagers' creates talents.It’s more obvious recently that the questions in competition have tended to thinking ability training and discovering universal laws rather than simple calculating skills. In this paper, forms of substitution methods are elaborated based on the examples of recent middle school mathematic competitions after generally description of the definition and traits of the method. The examples are sorted as simple and comprehensive ones to figure out the usage of substitution method. The features of the problems and practice principles are generally analyzed in the end as advice to middle school mathematics teaching and study.