Wireless Sensor Network is an open self-organized network, which faces serious challenge. Whole network can be split up into many separate subnets which cannot communicate with each other because some vital sensor nodes are attacked. A defense scheme which based on frequency hopping and fast network integration was proposed to react against split-network attack. Frequency hopping makes the communication frequency of the network escape from attack frequency while fast network integration makes the separate subnets reintegrate into a whole network in new communication frequency. Simulation results show the proposed scheme significantly reduces the success rate of attack and increases the lifetime of network.