During the process of deep excavation digging,it is an important task to monitor the excavation and affected building to make sure the spot's safety and go on wheels with the schedule.In this article some causation of slip and the calculation methods of freely setting station are first discussed,and then combined with the theory of polar coordinates a forecast method through distortion course curve analysis is put forward.The method with its low cost and speediness,can satisfy the precision requirement and provide the base data for safe evaluating and designing.