Recently, Mg₃(Sb,Bi)₂-based thermoelectric materials have received extensive attention owing to excellent thermoelectric properties and the low cost. This study investigates the change and mechanism of thermoelectric transport properties of Mg_3.275Mn_0.025Sb_1.49Bi_0.5Te_0.01/SiO₂ nanocomposite. The results show that nano-SiO₂ can effectively scatter phonons, promote the reduction of lattice thermal conductivity, and optimize the heat transport performance owing to the introduction of a large number of tiny grain boundaries. For example, when SiO₂ content is 0.54%, the thermal conductivity decreases by 15% from 1.24 W/(m·K) to 1.04 W/(m·K) compared with that of 0% SiO₂ sample at room temperature. At the same time, the material system also has a strong scattering effect on electrons. This leads to a sharp attenuation of power factor and electrical transport performance with decline of mobility and conductivity in the room temperature area. Nano SiO₂ is an effective candidate for regulating thermoelectric properties of Mg₃Sb₂ based thermoelectric material. The thermoelectric transport performance of the material will be improved by combining with other methods, such as appropriate grain boundary modification to reduce the potential barrier of charge carrier transport.