Abstract: In this study, the joining problem between the BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3-δ (BCZYYb) ceramic and Crofer 22 APU stainless steel in the protonic ceramic fuel cell stack was studied. The wettability of Ag-CuO braze on the surface of BCZYYb ceramic was explored. And the driving effect of the reactions between CuO and ceramic matrix on braze wetting was analyzed. The reactive air brazing process of Ag-CuO braze was studied, and the BCZYYb ceramic was soundly brazed to the stainless steel at 1010 ℃ for 20 min. The interfacial joining properties on both sides and the element distribution of joints were analyzed. It is showed that the wetting of Ag-CuO braze was promoted by the reactions between the CuO and BaO in the ceramic matrix. The braze could diffuse into the ceramic matrix, forming a thick fusion layer. The densification of the (Mn, Co)₃O₄ protective layer on the stainless steel could be densified by its reactions with the CuO from the braze, which played a key role in protecting the stainless steel from oxidization during reactive air brazing. Effects of CuO content on the microstructure and properties of joints were analyzed systematically. The highest shear strength of joints (21.6 MPa) was obtained by using Ag-2wt%CuO braze.