In　this　paper,　the　dynamic　response　of　underground　rectangular　fluid　tank　resting　on　an　elastic　foundation　and　subjected　to　arbitrary　dynamic　loads　is　developed　in　the　form　of　analytical　solution.　The　dynamic　responses　investigated　are　deflection,　bending　moment,　and　shear　force　of　the　tank.　The　underground　rectangular　tank　is　assumed　to　be　a　frame　composed　of　horizontal　and　vertical　beams　resting　on　an　elastic　foundation.　The　mechanical　resistance　of　elastic　foundation　is　modeled　using　spring　elements　that　account　for　soil　resistance　due　to　compressive　strains　in　the　soil.　The　fluid　in　the　tank　is　assumed　as　inviscid　and　irrotational,　and　the　influence　of　free-surface　wave　is　ignored.　An　analytical　solution　of　free　vibration　modes　of　empty　tank　(dry　modes)　is　derived　at　first,　and　then　the　free　vibration　modes　of　fluid　tank　infilled　with　water　(wet　modes)　can　be　obtained　by　the　dry　modes.　Based　on　the　wet　modes,　the　explicit　formulations　of　dynamic　responses　of　the　tank　are　finally　obtained　by　the　modal　superposition　method.　The　solutions　for　several　cases,　such　as　empty　tank　and　fully　filled　fluid　tank　subjected　to　harmonic　loads,　are　also　discussed.　Further　parametric　analysis　is　carried　out　to　investigate　the　influence　of　the　soil-structure　relative　stiffness　ratio　on　dynamic　responses　of　the　fully　filled　fluid　tank.