Based on an engineering example of intrusion-clearance of diaphragm wall, the mechanism and effect factors of intrusion-clearance are discussed by monitoring data and numerical analysis. The research result shows that the deviation of diaphragm wall, bad quality of improved soil and insufficient outward-moving of diaphragm wall cooperatively lead to intrusion-clearance, and the first has a significant effect. Moreover, dewatering of foundation pit also cause deformation of diaphragm wall, which accounts for more than 25% of the deformation during excavation, but usually despised in construction. The improving effect of reinforced soil attributes more to the deformation of diaphragm wall, especially the area closed to diaphragm wall. Some remedies and precautions are also given in this article, which could provide references of design and construction for similar projects.