In order to solve the problem that the weight of large scale theodolite's casting or steel plate welded structure is too heavy, lightweight design of large scale turntable based on topology optimization of continuum structure is investigated. Topology optimization could have better optimization efficiency because it can provide the optimized distribution of material and gain optimized structure in conceptual design phase of the structural design. In order to obtain lightweight and high stiffness structure, a truss turntable based on topology optimization of continuum structure is presented, and the validity of this lightweight structure is examined by finite element analysis. Comparing to the traditional welding steel turntable, the lightweight turntable, without changing the static displacement, can lose 386.48 kg(lightweight ratio is 26.7%), and its first-order natural frequency raises from 167.02Hz to 173.71Hz. Meanwhile, in other aspects this turntable also has better performance than traditional structure.