Abstract: Based on nonlinear simulation of progressive collapse of single-layer Kiewitt spherical shell,this paper studies the characteristics of deformation,load-bearing,and damage development of members,and presents a theory called three-phase that explains the progressive collapse mechanism very well in detail.Then,the development of deformation during the three phases is analyzed under different peak ground accelonations(PGA) and thus a new description of behavior level of spherical shell is set up with a damage assessment index based on relative displacement of gravity.All the work is useful for further study on the catastrophic mechanism of space structure under severe earthquake.