Aluminum alloy lattice structures featuring superior energy absorption, lightweight, and high strength-weight ratio, are achievable using Additive Manufacturing (AM) technology such as selective laser sintering. However, the size of the structure is limited, and the cost is relatively high. As a free-form rapid prototyping technology, Wire Arc Additive Manufacturing (WAAM) using electric arc as an energy source has the potential to fabricate medium to large-scale lattice structures with low cost and high process efficiency. This paper presents a strut-based process planning method for the WAAM system to fabricate large-sized lattice structures (strut-based wire structures) from the input models to the final parts. The proposed method includes the strut extraction module, strut bead modelling module, sequence initialization module, and sequence optimization module. Among these modules, bead modelling provides the essential database for process control, and an innovative sequence optimization module fulfills the automated process planning requirements without collision. A user-friendly interface has been developed for non-experts to operate the process planning. Finally, two- and three-dimensional lattice structures have been fabricated automatically using CAD models as inputs. These exercises demonstrate that the proposed strut-based process planning method contributes to producing practical lattice structures and highly automated WAAM system for industrial application.