MgAgSb is a promising room temperature thermoelectric material with relatively abundant element reserves for the construction of high-performance wearable thermoelectric batteries. In this study, Mg-Ag-Sb thin films were prepared on polyimide (PI) substrates by using magnetron sputtering, and the effects of annealing conditions on their thermoelectric properties were systematically investigated. The results showed that the MgAgSb flexible thermoelectric film composed of MgO, Ag₃Sb and Sb₂O₄ multiphases instead of pure phase, in which Ag₃Sb played main role of thermoelectric function. Different annealing atmospheres significantly improved the thermoelectric properties of MgO-Ag₃Sb-Sb₂O₄ (Mg-Ag-Sb) flexible thin films, among which the vacuum treatment appeared the best performance. Under vacuum conditions, the thermoelectric properties firstly increased and then decreased with the annealing temperature increasing, and the best thermoelectric property was achieved when the annealing temperature was 573 K, with a power factor of 74.16 μW?m^-1?K^-2 near room temperature. Moreover, the film exhibited good flexibility, and the conductivity changed by only 14% after 900 bending times. This study provides a reference for the preparation of MgAgSb flexible thermoelectric films and their wearable applications.