The invention provides an anti-freezing protection device and method for an air-conditioning unit water system. The anti-freezing protection device comprises a controller arranged in an outdoor unit, a water temperature sensor located in the air-conditioning unit water system and a first standby power supply. The controller is used for starting the first standby power supply to acquire the water temperature, collected by the water temperature sensor, of the air-conditioning unit water system when monitoring that an air-conditioning unit is in a powered-off manner, judging whether the water temperature is lower than or equal to a preset water temperature or not, and sending an anti-freezing alarm signal when the water temperature is lower than or equal to the preset water temperature. Due to the fact that the controller of the anti-freezing protection device starts the first standby power supply to acquire the water temperature, collected by the water temperature sensor, of the water system when the air-conditioning unit is powered off, and sends the anti-freezing alarm signal when the water temperature is lower than or equal to the preset water temperature so as to remind a user to perform anti-freezing treatment on the air-conditioning unit water system, safe and reliable guarantees are provided for stable operation of air-conditioning units with water as a cold medium.