已有建筑物中预制混凝土构件开裂是建筑结构中的常见缺陷。本文通过检测发现楼梯板的开裂是因构件反位浇筑，受力钢筋移位，有效高度减小而引起的。采用在板底加配体外钢筋并用横向加预应力的方法，可有效地控制裂缝，提高承载能力，满足使用要求。 Cracking of prefabricated concrete structures in existing buildings is a common defect in building structures. In this paper, it is found that the cracking of the staircase is caused by the anti-position of the member, the displacement of the reinforced rebar and the decrease of the effective height. Adopting the method of adding reinforcement outside the plate and adding prestressing force in the horizontal direction can effectively control the crack, improve the carrying capacity and meet the requirements of use.