Commonly　used　methods　for　identifying　modal　parameters　under　environmental　excitations　assume　that　the　unknown　environmental　input　is　a　stationary　white　noise　sequence.　For　large-scale　civil　structures,　actual　environmental　excitations,　such　as　wind　gusts　and　impact　loads,　cannot　usually　meet　this　condition,　and　exhibit　obvious　non-stationary　and　non-white-noise　characteristics.　The　theoretical　basis　of　the　stochastic　subspace　method　is　the　state-space　equation　in　the　time　domain,　while　the　state-space　equation　of　the　system　is　only　applicable　to　linear　systems.　Therefore,　under　non-smooth　excitation,　this　paper　proposes　a　stochastic　subspace　method　based　on　RDT.　Firstly,　this　paper　uses　the　random　decrement　technique　of　non-stationary　excitation　to　obtain　the　free　attenuation　response　of　the　response　signal,　and　then　uses　the　stochastic　subspace　identification　(SSI)　method　to　identify　the　modal　parameters.　This　not　only　improves　the　signal-to-noise　ratio　of　the　signal,　but　also　improves　the　computational　efficiency　significantly.　A　non-stationary　excitation　is　applied　to　the　spatial　grid　structure　model,　and　the　RDT-SSI　method　is　used　to　identify　the　modal　parameters.　The　identification　results　show　that　the　proposed　method　can　solve　the　problem　of　identifying　structural　modal　parameters　under　non-stationary　excitation.　This　method　is　applied　to　the　actual　health　monitoring　of　stadium　grids,　and　can　also　obtain　better　identification　results　in　frequency,　damping　ratio,　and　vibration　mode,　while　also　significantly　improving　computational　efficiency.