We　study　a　cross-cavity　quantum　Rabi　model　with　ultrastrong　coupling,　and　its　analytical　ground　state　is　demonstrated　to　be　approximately　obtained　by　using　the　rotating-transformation　method.　Our　method　requires　making　three　unitary　transformations　to　the　original　Hamiltonian　and　ignoring　the　three-party　cross　interaction.　In　near-resonance　and　ultrastrong-coupling　regimes,　the　ground-state　fidelity　is　higher　than　99%　for　a　+/-　20%　detuning　range.　We　analytically　demonstrate　that　the　entanglement　in　ground　state　first　quadratically　increases　with　the　qubit-oscillator　coupling　strength,　then　gets　close　to　the　maximum　value　1.　The　oscillator＇s　nonclassical　property　in　the　ground　state　is　well　explained　by　analytically　solving　the　Wigner　function.