The invention discloses a method for realizing a mobile IP service of flow based QoS (quality of service), involving a mobile IP user terminal AT, a RAN node, an access server or a foreign agent node and a Ha node. The method comprises the following steps that: (a) a flow based QoS information interface is added between the access server or the foreign agent and the HA node; (b) after QoS mapping relationship between up-link and down-link is established and maintained between the access server or the foreign agent and the AT and RAN nodes, the flow based OoS information containing the flow QoS mapping relationship is transferred to the HA node via the OoS information interface. The invention combines the characteristics of MIP service and transfers the flow based QoS information to the HA node by adding a message interface between the access server/ foreign agent node and the HA node, and thus the flow based OoS insurance scope of MIP service and the insurance system of the flow based QoS at the core network side are improved.