Background The reduction and fixation of Medial humeral calcar is difficult in the treatment of elderly proximal humerus Neer 3 and 4-part fractures with a single lateral locking plate. Our study investigated the efficacy of an intramedullary calcar supporting plate combined with a lateral locking plate for the treatment of 3- and 4-part fractures of the proximal humerus in the elderly through a deltoid splitting approach. Methods From June 2022 to December 2022, we treated six elderly patients with Neer 3 and 4-part fractures using proximal humeral intramedullary calcar support plate in combination with lateral locking plate through a deltoid splitting approach. Follow-up time was 6–12 months. Assessment indicators included fracture union, quality of reduction, and complication rate. The Constant-Murley score was used to record shoulder function at 6 months postoperatively. Results All 6 patients showed fracture union and anatomic reduction. Constant-Murley score was 79.5 (70–90) at 6 months postoperatively. There was no incision non-healing, internal fixation failure, bone non-union or surgical site infection, secondary surgery, or death. Shoulder impingement occurred in 1 case. Conclusion Proximal humeral intramedullary calcar support plate combined with lateral locking plate fixation through a deltoid splitting approach can effectively maintain fracture reduction, prevent inversion collapse of humeral head and internal fixation failure, and provide satisfactory clinical results at an early stage.