The prefabrication of BOSS requires multi-layer multi-pass welding with three-dimensional saddle shaped trajectories,which makes robot teaching and playback technology not suitable for actual use demands. Therefore an offline programming technology is developed, which allows users to self-define the arrangement of beads. By inputting the number of layers,the number of beads in every layer and the sectional area of single pass,all the welding trajectories can be generated automatically. Welding experiments indicate that this technology is suitable for BOSS prefabrication,which can greatly improve the welding quality and productivity.