The system of rainwater harvest ing for affovestation(SRHA) are analyzed based on soil water resources.The system focused on rainwater harvesting and improvement of rainwater use efficiently.Model of SRHA was established using water balance equation of the planted area to solve water shortage for afforestation in the arid and sem+iarid regions. Taking Tamarix ramosissima as an example, we explored main problems in the SRHA. The model has a reliable theoretical base and provides guidance for affroestat ion in the arid and sem+iarid regions.