The miniaturization of thermoelectric devices raises a strong demand 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 study, we adopted the acid pickling method to modify the surface structure of Bi_0.4Sb_1.6Te₃ materials 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 materials, which dramatically reduces the contact barrier height of Ni/Bi_0.4Sb_1.6Te₃ heterojunction from 0.22 eV 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 on the substrate surface and leading to a pinning effect. This greatly enhances the physical bonding between the materials surface and the Ni layer, which in conjunction with the metallurgical bonding formed by the interface diffusion reaction zone of about 50 nm thickness Ni/Bi_0.4Sb_1.6Te₃, greatly reinforces the interfacial bonding 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 × 4.9 mm² micro thermoelectric devices 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.