In order to design large-aperture mirror with light weight, high rigidity, high accuracy characteristics for the space telescope, a mirror topology optimization method based on the level set method was promoted. Firstly, a finite element model based on the 1-meter aperture mirror body initial structural model was constructed. The surface shape aberration RMS of the mirror was used as the objective function, which was constructed by the DRESP2 technique in Optistruct based on SIGFIT. With the weight constraint, the topology methods based on SIMP and level set method were used to optimize the structure separately during the design process. The separation threshold of the design result was studied based on the OSSmooth function. By analyzing the separation threshold of optimized model, the optimized exported structural model could be achieved. The number of elements with the middle density using the level set topology optimization design method is much smaller than level set method, and the connectivity of the structure is much better. The surface shape RMS value of the optimized model is smaller than λ/50(λ=632.8 nm), which satisfies the technique specification.