In water-scarcity areas fresh water is allocated with priority to urban areas and agriculture, and ecosystem function benefits are obtained from marginal quality water. Meanwhile the scientific use of saline water needs to quantify the effect of quality and quantity of water on plant growth. A saline irrigation experiment was carried out for two shelterbelt plants, Haloxylon ammodendron Bunge and Caragana karshiskii Kom, during 2007-08 in the arid region of northwest China. Three salt concentrations (3, 7 and 12g l -1) were considered and irrigation was controlled when soil moisture reached an enacted critical value. Based on the quadratic regressions fitted to the experimental data, the befitting irrigation with the three salt concentrations for H. ammodendron and C. karshiskii respectively were 381 and 295mm, 290 and 248mm, 213 and 188mm, averaged for two years. Two kinds of plant-water-salinity production functions, quadratic and square root functions were employed to describe the relationship between plant biomass increment and quality and quantity of applied saline water. The functions performed well with experimental data and showed a positive marginal productivity with water and a negative marginal productivity with salt for the frontal area. That could be useful for evaluating low-quality water exploitation and estimating the growth and yield of shelterbelt plants in connection with forest planning.