Through the tests of potted plants and measurement photosynthesis, this paper studies responses of photosynthetic character of slash pine's half-sib under water stress. There are 7 families compared with general slash pine (ab. CK). The net photosynthesis, stomatal conductance, CO2 concentration inside the cell, transpiration, water utilization efficiency, carboxylation efficiency and photosynthetic quantum yield are measured under different water strees (slight water stress while soil water content is 55% to 60%, moderate water stress while soil water content is 35% to 40% and heavy water stress while soil water content is 20% to 25%). The law of photosynthetic character with water stress is that Pn, Cond, Ci, Tr, WUE, Φ(subscript c) and CE of slash pine's half-sib drop with increased of water stress. The Pn, Ci, CE also drop line. It also shown that the wup go to up firstly, then drops after moderate water stress. The Cond and Tr tend to 0 with heavy water stress. The photosynthetic character value of slash pine's half-sib is better than that of general slash pine. It is one of the physiological causes with fast growth and high yield of slash pine's half-sib.