Abstract: A high-performance milling tool featuring an octagonal cutter composite arrangement was optimized to address the challenges of the long milling section, low milling speed and short single-trip footage caused by small wellbore size and high tubing strength in workover operations of ultra-deep slim holes. Based on a milling dynamics simulation model for P110 steel grade tubing, optimization design was conducted on the high-performance milling tool and the octagonal cutter was identified as the optimal cutting element, with 18° determined as the optimal back rake angle. During milling operations, the octagonal cutter leverages its high cutting engagement stress to effectively penetrate high strength tubing. Simultaneously, cutting stability is maintained by exhibiting the low coefficient of variation in cutting force fluctuation and low specific crushing energy. Additionally, the multistage composite layout utilizing exclusively octagonal cutters further synergizes the aggressiveness and stability of cutters, thereby maximizing energy utilization efficiency. Field tests demonstrated that compared to conventional mill shoes, the high-performance mill shoe increased single-trip milling footage by 32.86% and milling speed by 33.33%, yielding significant improvements in operational speed and efficiency. The optimization and application of the high-performance milling tool with octagonal cutter composite arrangement achieved both high stability and efficiency during the milling of high strength tubing, providing a novel and efficient milling tool for workover operations of ultra-deep slim holes.