The miniaturization of thermoelectric devices raises a strong requirement for the excellent interfacial properties of thermoelectric elements. Thus, achieving a heterogeneous interface with low interfacial contact resistivity and high interfacial bonding strength is a prerequisite for the successful fabrication of high-performance and high-reliability Bi₂Te₃-based micro thermoelectric devices. In this work, we adopt the acid pickling method to modify the surface structure of Bi_0.4Sb_1.6Te₃ material to synergistically optimize the interfacial properties of Bi_0.4Sb_1.6Te₃/Ni thermoelectric elements. The acid pickling process effectively modulates the work function of Bi_0.4Sb_1.6Te₃ material, which dramatically reduces the contact barrier height of Ni/Bi_0.4Sb_1.6Te₃ heterojunction from 0.22 to 0.02 eV. As a consequence, the corresponding interfacial contact resistivity of the element is greatly reduced from 14.2 to 0.22 μΩ·cm². Moreover, the acid pickling process effectively adjusts the surface roughness of the matrix, forming a V-shaped pit of 2–5 μm in depth on the substrate surface and leading to a pinning effect. This significantly enhances the physical bonding between the material surface and the Ni layer, which, together with the metallurgical bond formed by the interfacial diffusion reaction zone of about 50-nm-thick Ni/Bi_0.4Sb_1.6Te₃, greatly enhances the interfacial bond strength from 7.14 to 22.34 MPa. The excellent interfacial properties are further validated by the micro-thermoelectric devices. The maximum cooling temperature difference of 4.7 mm× 4.9 mm micro thermoelectric device fabricated by this process achieves 56.5 K, with hot side temperature setting at 300 K, and the maximum output power reaches 882 μW under the temperature gradient of 10 K. This work provides a new strategy for realizing the synergetic optimization of interfacial properties and opens up a new avenue for improving the performance of micro thermoelectric devices.