Wireless Sensor Network usually is deployed open environment to collect some sensitive information and has special features of its own are different from traditional network, which is vulnerable to internal and external attacks. Whole network can be split up into many separate subnets which cannot communicate with each other because some vital sensor nodes are attacked. This paper proposed an effective countermeasure based on ARMA prediction model and frequency hopping to react against split-network attack. ARMA model is used to evaluate the behavior of sensor nodes. Frequency hopping makes the communication frequency of the network escape from attack frequency. Then wireless sensor network is integrated into single network from split-network. Simulation results show the proposed countermeasure significantly reduces the success rate of split-network attack and increases the lifetime of network.