Perovskite　oxide　La0.9Sr0.1Ga0.8Mg0.2O3　(LSGM)　is　an　intermedium-temperature　solid　oxide　cell　electrolyte　material　with　extraordinary　oxygen-ion　conductivity.　However,　the　manufacturing　procedures　of　LSGM　discs　are　complex　involving　multiple　steps　of　powder　preparation,　forming,　and　sintering　at　high　temperatures.　Herein,　thin　LSGM　electrolyte　discs　are　prepared　by　coupling　of　tape　casting　and　in　situ　solid-state　reaction　using　oxides/carbonates　as　the　feedstock.　A　pure-phase　LSGM　electrolyte　disc　with　uniform　elemental　distribution　is　obtained　by　sintering　at　1450　degrees　C,　and　it　possesses　an　ionic　conductivity　of　0.105　S　cm(1)　at　800　degrees　C,　a　thermal　expansion　coefficient　of　12.2　x　10(6)　K-1,　and　a　bending　strength　of　156　MPa　A　170　mu　m　thick　LSGM　electrolyte-supported　single　cell　delivers　a　peak　power　density　of　0.96　W　cm(2)　at　800　degrees　C　and　an　electrolysis　current　density　of　1.82　A　cm(2)　at　1.5　V　with　no　noticeable　degradation　for　200　h.　The　findings　of　this　research　provide　a　cost-effective　approach　for　manufacturing　the　LSGM　electrolytes　of　efficient　and　durable　solid　oxide　cells.