Ammonia　is　a　carbon-free　hydrogen　carrier　with　high　volumetric　energy　density,　mature　infrastructure,　as　well　as　easy　and　safe　storage.　In　this　work,　we　design　a　good　cathode　material　based　on　spinel　NiMn2O4　with　an　oriented　structure　for　direct　ammonia　solid　oxide　fuel　cell　(DA-SOFC)　at　intermediate　temperature　(IT).　We　use　an　Mg-doped　strategy　at　A-site　to　induce　Ni2+　to　occupy　B-site　and　convert　Mn3+　to　Mn2+　or　Mn4+,　forming　a　disordered　but　stable　MgxNiMn2-xO4-delta　oxide　according　to　the　crystal　field　theory.　The　rich　Mn2+/Mn3+/Mn4+　pairs　over　MgxNiMn2-xO4　oxides　create　as　many　surface　oxygen　vacancies　as　possible,　and　serve　as　hopping　sites　for　hole　conduction　to　boost　the　interfacial　transportion.　Owing　to　richer　oxygen　vacancy　and　matchable　thermal　expansion,　Mg0.4NiMn1.6O4　(MNMO(0.4))　shows　higher　compatibility　with　SOFC　than　others.　An　anode-supported　Ni-YSZ　vertical　bar　YSZ　vertical　bar　MNMO(0.4)-YSZ　SOFC　can　obtain　a　peak　power　density　as　high　as　202　mW　cm(-2)　at　600　degrees　C　in　the　DA-SOFC　mode,　91%　of　that　in　the　H2SOFC　mode.　Compared　with　the　commercial　LSM-based　one,　the　MNMO(0.4)-based　DA-SOFC　exhibits　27%　higher　electrochemical　performance.　The　MNMO(0.4)-based　DA-SOFC　can　continuously　operate　at　0.5　V　and　800　degrees　C　for　more　than　50　h　without　remarkable　degradation,　revealing　a　promising　cathode　material　of　DA-SOFCs　in　IT-ranges.