The Mg2Sn thermoelectric films with a tiny metal Mg phase are prepared using a Mg-Sn alloy and a high pure metal Mg target by magnetron sputtering alternately. The phase composition, surface and cross-sectional morphology, element content and distribution of the deposited films were studied by X-ray diffraction (XRD) pattern, scanning electron microscope (SEM), and energy diffraction spectrometer (EDS). The resistivity and Seebeck coefficients of the deposited films were measured by using the Seebeck coefficient/resistance analysis system LSR-3. And the power factor of the Mg2Sn film with different Mg contents was studied. The XRD shows that the deposited film is composed of nano-sized Mg2Sn phase with cubic anti-fluorite structure and a small number of nano-sized metal Mg phase. The content of nano-sized metallic Mg phase increases with increasing Mg target sputtering time, and the resistivity and Seebeck coefficient increase first and then decrease, which is due to the phase interface between the nano-sized metallic Mg phases and substrate phases.The nano-sized metal Mg phase with proper quantity exists in the Mg2Sn film, which is beneficial to improve the Seebeck coefficient. The deposited Mg2Sn films containing proper quantity nano-sized metallic Mg phase possess a higher power factor due to their high Seebeck coefficient and an appropriate resistivity. The layered Mg2Sn film can significantly increase the Seebeck coefficient and the power factor of the film is significantly improved even if the resistivity increases.