The invention discloses a Bi3Se4 nanobelt and an application of the Bi3Se4 nanobelt as a thermoelectric material, a preparation method of the Bi3Se4 nanobelt comprises the following steps: 1)adding selenium powder in liquid paraffin or ODE, magnetically stirring, heating to the temperature of more than 190 DEG C, and fully dissolving the selenium powder to prepare a Se precursor solution; 2)adding Bi2O3 in the liquid paraffin or ODE, adding oleic acid, heating to the temperature of more than 230 DEG C, and dissolving Bi2O3 to prepare a Bi precursor solution; and 3)rapidly drawing the Se precursor solution off by an injection syringe and adding in a Bi precursor solution, and keeping the mol ratio of Bi/Se of 3:4, stirring at temperature of 230-300 DEG C and reacting for 15s-30min, then placing in methanol for a quenching reaction, standing to form flocculent deposition, and centrifuging to obtain the Bi3Se4 nanobelt. The prepared Bi3Se4 material has a nanobelt shape structure, and can be taken as the thermoelectric material. The preparation method has the characteristics of simple equipment, low cost, environmental protection, excellent performance of products and the like, and provides an operable way of synthesizing Bi3Se4 nanobelt with high quality.