Perovskites　decorated　with　catalytically　active　nanoparticles　show　promising　potential　in　power　generation　and　energy　conversion.　Here,　with　the　aim　of　developing　symmetrical　solid　oxide　fuel　cells　(SSOFCs)　possessing　high　performance,　an　ingenious　approach　of　in　situ　exsolution　of　metallic　Ru　nanoparticles　under　SSOFC　operating　condition　is　proposed.　In　this　study,　a　novel　perovskite　oxide　with　A-site　deficiency　Sm0.70Sr0.20Fe0.80Ti0·15Ru0·05O3−δ　(SSFTR7020)　is　meticulously　designed　and　successfully　synthesized.　At　800　°C,　the　polarization　resistance　(Rp)　of　SSFTR7020　cathode　is　only　0.13　Ω　cm2,　since　the　adidtionally　formed　oxygen　vacancies　contribute　to　the　oxygen　reduction　reaction.　Moreover,　it　is　found　that　numerous　metallic　Ru　nanoparticles　are　exsolved　from　SSFTR7020　perovskite　support　upon　reduction,　while　no　partilcle　can　be　observed　for　stoichiometric　SSFTR　perovskite.　This　is　because　the　artificially　introduced　deficiency　can　make　exsolutsion　become　more　dynamic.　The　maximum　power　output　of　symmetrical　cell　SSFTR7020|Sm0.2Ce0·8O1.9　(SDC)|SSFTR7020　reaches　476　mW　cm−2　after　operation　~96　h　at　800　°C　in　humidified　hydrogen.　©　2020　Elsevier　B.V.