Taking the sizes of cross sections and the volume of ordinary spherical nodes as design variables and overall cost as objective function, the size optimization design of spatial lattice shell is developed by means of two-level algorithem based on discrete variables.The credible structural analysis programme named Algor(Super SAP93) is combined to accomplish the finite element analysis of mathematical model. In addition, zero-step modification is considered to improve the computational precision. At the end, through two numerical examples, the theories above are tested and verified to be correct. Compared with full stress optimization method, the computational results are satisfactory.