In SDN networks, the abstraction of the data forwarding plane is the precondition of the formulation of the southern interface protocols, such as OpenFlow. Currently, the standard OpenFlow protocol assumes that each flow table item of the data plane is composed of defined rules and corresponding action sets. FlowMod messages of the data plane carry these rules and action sets. However, with the continuous development of SDN application scenarios, the limited and predefined actions of the OpenFlow protocol make it difficult to meet some new configuration requirements, such as centrally controlled segment routing management configuration, SDPA configuration and so on. To improve the flexibility of action definition in SDNs, and remove the restrictions on actions, we firstly propose a new action extension mechanism, called flexible action extension (FAE). The FAE bases on FlowMod messages in OpenFlow, and can satisfy the needs for userdefined actions which can extend in a flexible and simple way. And then we implement the FAE useing open source Floodlight controller and FAST switch, and experiments show that the FAE can effectively support the extended actions in centrally controlled segment routing management configuration.