In the process of the foundation pit excavation of a freezing room with 810 meters depth, which is in a large foundation pit in coastal soft soil area, collapse caused by anchor bolt failure occurred. By the analysis and calculation comparisons of simulation working conditions element internal force in 5 aspects: insufficient consideration of digging depth, bad geological environment, bolt spacing exceeding the design requirements, insufficient range of step-slope by soil dumping and taking higher value of anchor pullout capacity, it was concluded that the increased anchor tension design value greatly exceeded the anchor anti-pulling force capacity by the comprehensive influence of above 5 aspects, which led the anchor bolt failure and the instability of foundation pit.