This paper studies the impact of buyer guarantee mechanism on all members in a supply chain， which is consist of a capital-constrained supplier and a capital-rich buyer. It is found that if the synthetic financing rate is equal to the financing rate in the traditional supplier financing，the buyer bears the financing risk instead of the bank and there is no possibility that the bank and the buyer can both benefit from the buyer guarantee mechanism; if the synthetic financing rate is lower，a Pareto improvement where all members can benefit from the buyer guarantee mechanism exists under certain circumstances. Otherwise，Pareto improvement cannot be achieved.