The　experimental　observation　of　quantum　phase　transitions　predicted　by　the　quantum　Rabi　model　in　quantum　critical　systems　is　usually　challenging　due　to　the　lack　of　signature　experimental　observables　associated　with　them.　Here,　we　describe　a　method　to　identify　the　dynamical　critical　phenomenon　in　the　quantum　Rabi　model　consisting　of　a　three-level　atom　and　a　cavity　at　the　quantum　phase　transition.　Such　a　critical　phenomenon　manifests　itself　as　a　sudden　change　of　steady-state　output　photons　in　the　system　driven　by　two　classical　fields,　when　both　the　atom　and　the　cavity　are　initially　unexcited.　The　process　occurs　as　the　high-frequency　pump　field　is　converted　into　the　low-frequency　Stokes　field　and　multiple　cavity　photons　in　the　normal　phase,　while　this　conversion　cannot　occur　in　the　superradiant　phase.　The　sudden　change　of　steady-state　output　photons　is　an　experimentally　accessible　measure　to　probe　quantum　phase　transitions,　as　it　does　not　require　preparing　the　equilibrium　state.　©　2024,　The　Author(s).