The　secure　sum　protocol　is　a　useful　basic　protocol　of　Secure　multiparty　computation　(SMC).　And　it　has　numerous　applications.　However　traditional　secure　sum　protocol　can　not　guarantee　the　fairness.　In　addition,　most　previous　protocols　can　not　resist　the　collusion-attack.　This　paper　proposes　a　collusion-free　rational　secure　sum　protocol　in　which　we　combine　game　theory　with　the　multiparty　secure　sum　protocol.　In　the　setting　of　rational　secure　sum　protocol,　the　gain　of　following　the　protocol　is　more　than　the　gain　of　deviating,　and　no　player　of　the　coalition　parties　can　do　better,　even　if　the　whole　coalition　parties　cheat.　Analysis　shows　that　the　protocol　can　resist　the　collusion　attack　with　at　most　n-2　players,　and　rational　players　have　to　abide　by　the　protocols.　Unlike　previous　secure　sum　algorithms,　this　paper　aims　at　obtaining　complete　fairness　even　though　without　a　majority　of　honest　parties.