As a newly emerging financing method, BT (Build-Transfer) Mode has demonstrated its extensive application in infrastructure construction. In allusion to the principle of BT project risk allocation, this paper first provided an analysis on two kinds of risks within BT Mode: the risk as common construction mode does and certain particular risks of its own. And then, based on the principle of “risk sharing and benefit sharing”, it established the mathematical model of risk sharing. The model result shows that the project's maximum overall efficiency will be achieved when the participant who has the largest risk preference coefficient bears the project risk. Finally, according to the conclusion, it puts forward specific principles of the apportionment of risks and benefits, which can serve as practical guidelines for each participant during BT project negotiation.