Spin　squeezing　is　one　of　the　most　attractive　methods　for　realizing　high-precision　metrology.　In　this　paper　we　propose　a　protocol　for　generating　spin　squeezing　in　an　atomic　ensemble　via　a　fully　quantum　degenerate　parametric　amplifier.　We　discuss　the　properties　of　generating　spin　squeezing　with　and　without　driving　the　pump　cavity.　Numerical　simulation　results　show　that　the　generated　spin　squeezing　strength　is　sizable　and　is　comparable　to　that　obtained　using　a　two-axis　twisting　model.　Moreover,　we　demonstrate　that　the　protocol　is　experimentally　feasible　by　introducing　the　corresponding　experimental　parameters.　Therefore,　the　proposed　protocol　provides　a　promising　approach　to　realize　spin　squeezing　in　photon-spin　coupling　systems.