The invention discloses a device and method for treating cold feedwater outside a converter station. The device comprises a pipe mixer for mixing raw water and a water purification agent, and a water purifier. The device also comprises an ion exchanger. The water purifier is connected to an outlet of the pipe mixer. The outlet of the water purifier is connected to a middle water tank. An outlet of the middle water tank is connected to the top end of the ion exchanger by a lifting pump. The bottom of the ion exchanger is communicated with a cold water feeding pipe outside a converter station. The device produces high quality product water, prevents scaling in a pipe and has a high raw water treatment automation degree.