In recent years, the cathode materials of magnesium ion batteries have become a hot point of research, and the improvement of high-rate performance and cycle stability has become the main research goal. In this paper, sodium manganese oxide (Na_0.55Mn₂O₄·1.5H₂O) nanomaterial with a blended structure of nanowires and nanosheets is prepared by the hydrothermal method. The structure and morphology of the material are analyzed by X-ray diffraction and scanning electron microscopy. The variable rate charge-discharge curves and variable scan rate cyclic voltammetry curves are obtained by a battery tester and electrochemical workstation, respectively. The results show that the hydrothermal reaction time has significant effects on phase structure and morphology composition of the material. The nanosheets and nanowires in the sample form a closely blend by 72-h hydrothermal reaction (NMO-72), and the nanosheets effectively fill into the intersecting space of the nanowires. In this way, the tap density of the material is improved. More importantly, NMO-72 has higher discharge specific capacity and rate cycling performance. At a current density of 50 mA·g^–1, the discharge specific capacity of NMO-72 reaches 229.1 mAh·g^–1. At a current density of 1000 mA·g^–1, the discharge specific capacity of the NMO-72 stabilizes at 81 mAh·g^–1. When the current density returns to 50 mA·g^–1 again, the discharge specific capacity remains stable at 164.7 mAh·g^–1. Besides, the cyclic voltammetry test shows that the NMO-72 material has more excellent magnesium ion diffusion kinetic performance than other materials. Therefore, the NMO-72 material has more excellent reversible specific capacity, high rate performance and cycling stability.