The invention discloses a caustic soda system emergency PLC control system and process. By selecting key process index points of the system, a relay and a switch button are used for switching betweena DCS control system and a PLC control system in the system fault state, a PLC control cabinet and a PLC control computer control operation of all key process index points and equipment in the systemfault state; and the DCS control cabinet and a DCS operation computer control operation of all key process index points and equipment in the system normal state. In order to solve the problem that a caustic soda system cannot be halted due to DCS system failures in the normal production process, the key process index points of the system are scientifically selected, the emergency PLC system is designed, the safe parking under the DCS system fault state is achieved, and personnel safety and equipment safety in the system production process are guaranteed.