In order to meet the design requirements of a Long Slit Spectrograph(LSS) worked at the World Space Observatory,the LSS primary structure was designed and a structure optimization method for the LSS primary structure was studied. An optimization model of the LSS primary structure was established by taking the basic frequency as the constrained conditions and the total mass as a objective function,and an Ant Colony Optimization(ACO) algorithm with an improved pheromone updating rule was put forward. Based on the improved ACO algorithm and the Finite Element Analysis(FEA) software ANSYS,a combined optimization design method was presented to solve above structural optimization problems. The optimized structure obtained from the combined method has reduced about 25% in mass as compared with that of the original design and by 7% that of the zero-order optimization method provided by the Design Opt module of ANSYS.Experimental results show that the optimal primary structure satisfies the design requirements. The combined method takes advantages of the characteristics of the ACO algorithm and software ANSYS and has potentials to be applied to other similar engineering structural optimizations.