ObjectiveTo investigate the feasibility and effectiveness of using a modified latissimus dorsi musculocutaneous flap for the treatment of upper limb injuries with complex tissue defects classified as Gustilo-Anderson type ⅢB and ⅢC. Methods Between January 2021 and January 2024, 19 cases of upper limb injuries with complex tissue defects classified as Gustilo-Anderson type ⅢB and ⅢC were repaired with a modified latissimus dorsi musculocutaneous flap. There were 14 males and 5 females, with an average age of 33 years (range, 8-65 years). Among these cases, 12 involved injuries to the upper arm and elbow, while 7 involved injuries to the forearm. The disease duration ranged from 2 hours to 10 days (mean, 2.6 days). The patients underwent emergency internal fixation or external fixation for fractures, along with repair of neurovascular injuries. Subsequently, based on the type and extent of tissue defect, 11 cases were treated with pedicled flap transplantation, while 8 cases underwent free flap transplantation. The size of the muscular flaps ranged from 16.0 cm×10.0 cm to 24.0 cm×14.5 cm, while the size of skin flaps ranged from 12.0 cm×5.0 cm to 27.0 cm×7.5 cm. All incisions at donor sites were sutured. Results All 19 flaps survived completely, with primary wound healing in 17 cases and secondary healing with skin grafting on the muscular flap in 2 cases. The incisions at donor sites healed by first intention. All patients were followed up 10-41 months (mean, 21 months), and limb salvage was successful in all cases. All fractures healed with a healing time of 8-14 weeks. At last follow-up, according to the functional evaluation criteria for upper limb replantation established by the Hand Surgery Branch of Chinese Medical Association, 15 cases demonstrated excellent functional recovery, while 3 cases showed good functional outcomes, 1 case showed poor recovery. Conclusion The modified latissimus dorsi musculocutaneous flap, through partial separation of the latissimus dorsi muscle and musculocutaneous perforators combined with a staggered arrangement of the muscle and skin components, enables flexible reconstruction of defects at different tissue levels. This technique demonstrates advantages including a high flap survival rate, satisfactory limb salvage outcomes, favorable functional recovery, and relatively low donor site morbidity, making it an effective method for reconstruction of Gustilo-Anderson type ⅢB and ⅢC upper limb injuries with composite tissue defects.