在土木工程专业本科教学中，斜截面投影长度的计算是一个难点，也是一个重点。简支梁抗剪承载力复核时，首先必须要确定斜截面顶端的位置，即确定斜截面的投影长度。对现有的多种斜截面投影长度的计算方法进行了比较分析，并介绍了一种新的计算方法，并通过算例验证了其正确性。新计算方法概念清晰、计算简单，便于学生掌握和理解抗剪承载力复核计算的相关内容，对该部分内容的教学改革具有积极的意义。实践表明，新方法在木工工程专业本科教学实践中取得了良好的教学成果。

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.