Metallic nanoparticles structured perovskite oxides prepared by the in situ exsolution method are widely utilized as alternative anodes for solid oxide fuel cells. In this work, Sr_xFe_1.3Ni_0.2Mo_0.5O_6−δ (SFNMx, x = 1.90, 1.95, 2.00, and 2.05) materials are prepared to construct Ni–Fe alloy nanoparticles structured SFNMx anodes. It is found that the microstructure as well as electrochemical activity of SFNMx anodes can be successfully manipulated by altering A-site Sr non-stoichiometry. Moreover, the electrochemical performance of the symmetrical cells with SFNMx electrodes demonstrates that A-site Sr deficiency can effectively accelerate the exsolution of Ni–Fe alloy nanoparticles from the parent oxides, reasonably providing more active sites for the hydrogen oxidation reaction and effectively lowering the electrode polarization resistance to 1.04 Ω cm² with decreasing Sr content to 1.95, which is in good agreement with the results predicted by the regular-solution model. In addition, the distribution of relaxation times analysis results indicate that H₂ adsorption/dissociation/ionization, which can be strongly accelerated by in situ exsolved Ni–Fe alloy nanoparticles, is the predominant rate-limiting step. It is concluded that A-site non-stoichiometry in perovskites can greatly facilitate the exsolution of metallic nanoparticles, and enable effectively enhanced electrochemical activity. Our findings can guide the development of nano-architectures of A_1−xBO₃ materials for other energy conversion and storage devices.