Abstract: MnO₂ with different morphology and crystal structure were prepared via a low-temperature hydrothermal method in stainless-steel autoclave at 120℃ and different dwell time, which （NH₄）₂S₂O₈ is oxidizing agent and MnSO₄ is Mn source. The products were characterized by powder X-ray diffraction （XRD）, scanning electron microscopy （SEM） and transmission electron microscope （TEM）, and the electromagnetic properties and microwave loss mechanism of as-obtained MnO₂ were firstly reported. The results show that the dwell times play a crucial role in the phase purity, the crystal structure, the morphology and the electromagnetic properties of the products. The as-prepared MnO₂ transformed gradually from γ-MnO₂ to α-MnO₂ and β-MnO₂ and became finally a pure tetragonal phase of β-MnO₂, and the morphology changed also from microspheres with flocculus on the surface into dispersive nanorods with the increase of the dwell times from 1h to 48h. The real and imaginary parts of the complex permittivity of as-prepared MnO₂ increased orderly with rising dwell times, and the complex magnetic permeability were on the contrary. Lastly, the microwave loss mechanism of as-obtained MnO₂ was analysed.