In-plane heat dissipation technology based on flexible thermoelectric film cooling is expected to provide a solution to efficient in-plane heat dissipation of electronic devices. However, the low electrical transport performance of flexible thermoelectric films and the difficulty in designing the structure of in-plane heat dissipation device seriously restrict the applications of this technology in heat dissipation of electronic devices. In this work, an epoxy/Bi_0.5Sb_1.5Te₃ flexible thermoelectric film is incorporated with graphene which can simultaneously regulate the electrical and thermal transport behaviors. It is found that the incorporating of graphene not only contributes to the preferential orientation of Bi_0.5Sb_1.5Te₃ grains along (000l), but also provides a fast carrier transport channel. The carrier concentration and mobility of graphene/Bi_0.5Sb_1.5Te₃ flexible thermoelectric film are simultaneously increased. Comparing with the epoxy/Bi_0.5Sb_1.5Te₃ flexible thermoelectric film, the highest power factor of the flexible thermoelectric film with 1.0% graphene at room temperature reaches 1.56 mW/(K²·m), increased by 71%, while the cooling temperature difference is doubled. Using this high-performance graphene/Bi_0.5Sb_1.5Te₃ flexible thermoelectric film cooling, a cascade structure high-efficiency in-plane heat dissipation device is designed and fabricated. The device can dissipate heat from the heat source area to the heat dissipation area step by step and reduce the temperature of the heat source area by 1.4–1.9 ℃, showing an efficient and stable in-plane heat dissipation capability.