To investigate the effect and mechanism of AUF1 regulating p21 on the proliferation and apoptosis of osteosarcoma cells. Methods qRT-PCR and Western blot were used to detect the expression level of AUF1 in osteosarcoma tissues and cells. After transfection of sh-NC and sh-AUF1 into U2OS and HOS cells, CCK8 was used to detect cell proliferation. qRT-PCR was used to detect the expression level of proliferation-related molecule PCNA. The flow cytometry was used to detect cell apoptosis. Western blot was used to detect the expression level of apoptosis-related molecule Cleaved-caspase-3. RIP and RNA half-life experiments were used to measure the mechanism of AUF1 regulating p21. Results The expression level of AUF1 mRNA and protein in osteosarcoma tissues and cells was significantly higher than that in normal tissues adjacent to cancer (P＜0.05), and the expression level of AUF1 mRNA in osteosarcoma patients with metastasis was significantly higher than in patients without metastasis (P＜0.05). U2OS and HOS cells were transfected with sh-NC and sh-AUF1, respectively. The expression levels of AUF1 mRNA and protein in the sh-AUF1 group were significantly lower than those in the sh-NC group (P＜0.05). Low expression of AUF1 could significantly inhibit the proliferation of U2OS and HOS cells and the expression of proliferation-related molecule PCNA (P＜0.05), and promote cell apoptosis and the expression of apoptosis-related molecule Cleaved-caspase-3 (P＜0.05). Low expression of AUF1 significantly promoted the expression of p21 mRNA and protein in U2OS and HOS cells (P＜0.05). AUF1 protein bound to p21 mRNA (P＜0.05), while low expression of AUF1 significantly inhibited the degradation rate of p21 mRNA (P＜0.05). Conclusion AUF1 was highly expressed in osteosarcoma tissues and cells. The low expression of AUF1 could inhibit the degradation of p21 mRNA and increase the protein expression of p21, thereby inhibiting the proliferation of osteosarcoma cells and promoting cell apoptosis