Sea workover machine is used as the necessary equipment for the whole workover operation, its derrick is the most critical bearing part of workover machine, with large load and serious accidents are often caused due to insufficient strength and stability. Based on the static performance of the derrick is mainly used as the design criterion, and it does not consider the small modularity and lightweight requirements of the derrick. There is a huge material waste only by adding the materials of the rod material to ensure the safety and reliability of the derrick. So it provides very important engineering significance while the safety and reliability of the derrick are ensured, in order to realize the lightweight design of offshore workover rig and reduce the material consumption. The simulation annealing algorithm was used to investigate the lightweight of the derrick. The results show that the industry standard is met by the statics, dynamics and stability of the lightweight derrick. It can be seen that the accuracy of light weight study of offshore workover rig is high. The theoretical basis and data reference can be provided by the research methods and results for the structural optimization of derrick of workover rig.