The　code　sharing　is　the　popular　and　effective　cooperation　of　the　airlines.　First,　this　paper　applies　the　spokes　model　to　describe　the　different　preferences　of　passengers　for　three　airlines,　and　proposes　the　twostage　game　model　and　the　simple　method　for　the　code　sharing　agreement　choosing　stage　and　the　tickets＇　pricing　stage　under　the　free-sale　model.　Second,　we　use　the　numerical　example　to　show　the　feasibility　of　the　proposed　models　and　method.　By　comparison　and　analysis,　the　Nash　equilibrium　situations　of　the　two-stage　game　can　be　obtained.　This　study　demonstrates:　The　code　sharing　agreement　saves　the　total　operating　cost,　but　it　raises　the　average　airfare　of　the　aviation　market,　so　that　the　airlines＇　profits　increase　and　the　passengers＇　surpluses　decrease;　the　government　should　promote　the　realization　of　better　Nash　equilibrium　situation　in　order　to　maximize　the　total　social　welfare;　to　further　improve　the　total　social　welfare　during　the　period　of　Corona　Virus　Disease-19,　the　government　could　give　the　airlines　some　cost　subsidies　or　adjust　the　passengers＇　psychological　costs.　Therefore,　this　paper　provides　significant　theoretical　and　methodical　supports　to　optimize　the　choices　of　the　code　sharing　agreements　and　to　improve　the　total　social　welfare　in　the　aviation　market.　©　2022,　Editorial　Board　of　Journal　of　Systems　Engineering　Society　of　China.　All　right　reserved.