The invention discloses an automatic control system and method for rainwater collecting and water-saving irrigation of a greenhouse. The system comprises a greenhouse rainwater collecting system, a pipeline system and a water-saving irrigation automatic control system. The greenhouse rainwater collecting system comprises a rainwater collecting face located at the top of the greenhouse, confluence grooves formed in the lower end of the rainwater collecting face and a rainwater collecting pool arranged on the ground or underground. The rainwater collecting pool is communicated with the confluence grooves through water guide pipes. The pipeline system comprises a water delivery and distribution pipeline and capillary pipes. The water-saving irrigation automatic control system comprises a direct current power supply, a central intelligent controller, soil moisture sensors arranged in soil, a water level sensor arranged inside the rainwater collecting pool, and a water pump arranged at the bottom end of the rainwater collecting pool. By means of the automatic control system and method for rainwater collecting and water-saving irrigation of the greenhouse, a water resource guarantee is provided for facility agriculture production, a technical means is provided for water-saving irrigation automation, and meanwhile automatic control over different irrigation water quotas is achieved.