During undergraduate teaching process of civil engineering, calculation of projection length of diagonal section is a difficulty and also a key content. Defining the top position of the diagonal section is the first step when checking the simply supported beam’s shear capacity, namely, confirming projection length of diagonal section. Current textbooks explained various methods about calculation of projection length of diagonal section. We compared and summarized current calculation methods and presented a new accurate method which had been verified. The concept of the method is clear and the calculation is simple. Therefore, it is convenient for students to understand and master the content related to check shear capacity of diagonal section and has positive significance for teaching reform. Practice shows that the method achieved effective results in undergraduate teaching of civil engineering.