Abstract:Two years old early-fruiting walnut seedling were water stress in early July. As the water stress increased, net photosynthesis rate of the leaves decreased from the maximum state slowly first than, rapidly later on, and ultimately completely inhibited. The relations of net photosynthesis and leaf water potential soil water content, leaf water content, stomatal diffusion resistance, and the angle between the first couple leaflets at the top of compound lesf were studied and discussed in this paper. It was shown that the angle and the leaf water potential were the better indexes to indicate the water state of trees than soil water contant. When the angle was more than 90° and the leaf water potential was more than- 1.20MPa, the net photosynthesis rate of leaves could be maintained at higher level. Whereas leaf water content and stomatal diffusion resistance could not be used as the water indexes for early-fruiting walnut. Critical water potential of early-fruiting walnut for stomatal closure was-1.35MPa.