Purpose This study investigated the efficacy of nanohydroxyapatite- (nHA-) coated biological prosthesis combined with platelet-rich plasma (PRP) in hemi hip replacement of femoral neck fracture (FNF) in the elderly. Methods From September 2018 to September 2021, 102 elderly patients with FNF treated in our hospital were chosen and divided into two groups according to different intervention methods. Fifty-one patients in the bone cement group were treated with bone cement prosthesis, and the rest 51 patients in the observation group were treated with nHA biological prosthesis combined with PRP in hemi hip replacement. In order to explore the osteogenic effect of nHA and PRP, osteoblasts were cultured. Results It was found that nHA and PRP could both effectively promote the proliferation of osteoblasts and improve their mineralization ability, especially when used in combination. In the course of clinical therapy, we found that the use of biological prosthesis combined with PRP could effectively reduce the level of serum procollagen type I carboxy terminal peptide (PICP) and better improve the levels of bone alkaline phosphatase (BALP) and bone Gla protein (BGP), so as to reduce the bone conversion rate and promote the formation of new bone around the prosthesis. In addition, no significant difference was found in intraoperative bleeding, operation time, hospital stay, 48 h drainage volume, partial weight-bearing time, and complete weight-bearing time between two groups. Otherwise, the use of biological prosthesis could effectively avoid the occurrence of adverse reactions such as bone cement crisis and fracture around femoral prosthesis, so as to better restore the hip function and improve patients' life quality. Conclusions Therefore, in hemi hip replacement of FNF in the elderly, nHA biological prosthesis combined with PRP can effectively promote the formation of new bone around the prosthesis stem, so as to obtain good initial stability, enable patients to carry out early weight-bearing exercise, and effectively avoid adverse reactions caused by bone cement prosthesis, thus improving patients' hip function and life quality.