Abstract:This article is making an effort to find out the obstacles to the mastery of the definition of high school function. The findings show that the definition of function contains a large number of nonessential attributes and symbols, which adds to the difficulty for the students to form the concept of function; the definition involves many highly abstract symbols, which will increase students’ difficulty in understanding the definition. At the same time, the definition embeds much profound mathematical thoughts, the mastery of which requires a long-term understanding and experience, which poses challenges to both the teaching and learning of it. The chunking needs to process the information in the definition of function exceed the up- per capacity limit of working memory load by far and the study of the definition of function can not proceed in the way of most economic assimilation, etc.