High school mathematics knowledge system is complex, and the study time in class is limited, so for us, the best way to eliminate the strangeness of knowledge is to preview. Preview before class can make us quickly enter the learning state, so as to absorb and understand knowledge efficiently. Obviously, many students understand this truth, but because there are many subjects to learn in high school and a lot of homework to do after class, the time left for preview is very limited. If every subject is previewed according to the teacher's requirements, there will inevitably be favoritism. In this regard, the author believes that in order to ensure the efficiency of preview, we must be targeted, that is, we must start with the key points and difficulties and combine our own learning situation to preview, so as to better eliminate the ability difference between ourselves and other students and fundamentally overcome the difficulties in mathematics learning in senior high school. For example, when learning "Algorithm and Program Diagram", we should preview the knowledge of this chapter, make clear the design principle of program diagram, and how to judge the two branches of "yes" and "no", which is the knowledge focus of this course. In the process of preview, we should make assumptions and understandings about knowledge from our own thinking, and think and revise our preview in class, which is the performance of efficient preview.