Additive manufacturing based on robotic welding is used for the manufacture of metal parts by applying an arc as a heat source and wire as feedstock. The process is known as wire arc additive manufacturing (WAAM). However, the current WAAM process has a limitation in fabricating block structure components with high geometry accuracy and consistent welding due to the process complexity and the lack of appropriate process planning methods. Furthermore, common defects such as voids, gaps, and collapse decrease the mechanical properties of the final product. This paper presents a novel process planning method based on a mixed heat input (MHI) strategy to minimize voids and collapse defects that occur in fabricating large block structure components while maintaining a high manufacturing efficiency. By separating each layer into boundary layers and inner layers, the MHI method applies various heat input conditions at different positions of the layer allowing the construction of defect-free components. The performance of this method is shown in a validation study considered as the fabrication of a regular structure component. To evaluate the mechanical properties of the deposited sample, the hardness and microstructure are reported and compared with the conventional WAAM process. Then, the MHI strategy is applied to the manufacturing of a large block structure component with a similar structure as the validation study. Furthermore, the robustness of the proposed MHI strategy in fabricating complex geometric parts is evaluated by two case studies. The results show that such strategy succeeds in achieving high production efficiency and quality simultaneously.