We　propose　a　protocol　to　study　the　dynamical　Casimir　effect　(DCE)　in　an　optomechanical　system　involving　a　frequency-modulated　resonator　with　a　vibrating　mirror.　The　frequency　modulation　changes　the　resonant　condition　of　the　photon-phonon　parametric　coupling　and　then　makes　the　photons　generated　by　the　DCE　become　more　observable.　Through　the　vacuum　Casimir-Rabi　splitting,　the　generation　of　photons　from　the　DCE　is　demonstrated.　Meanwhile,　numerical　simulations　show　that　when　driving　the　mechanical　mode,　the　system　is　able　to　emit　observable　photon　fluxes,　even　though　the　resonator　frequency　is　much　larger　than　the　mechanical　frequency.　The　results　also　validate　the　protocol　is　robust　to　the　resonator　decay　and　the　modulation　parameters.　Thus,　our　protocol　paves　the　way　to　explore　the　DCE　for　cavity　optomechanical　systems.　(c)　2025　Optica　Publishing　Group　under　the　terms　of　the　Optica　Open　Access　Publishing　Agreement