Abstract: In order to explore the important role of irrigation and fertilization for rainfall-harvesting agriculture on improving the niche-fitness and increasing productivity of spring wheat, the effects of different water supply and fertilizer treatments on the leaf area index, root biomass, niche-fitness and productivity were studied by the field plot experiment and modeling method. The results showed that impacts of water supply and fertilizer treatments on leaf area index (LAI) and root biomass were dramatic. Compared rich water treatment (RW), medium water treatment (MW) and nature water treatment (NW) with low water treatment (LW), the maximum LAI in anthesis increased by 56.9%, 30.9% and 19.1%, and the mean of root biomass increased by 84.2%, 50.4% and 26.6%, respectively. Meanwhile, these indices were also increased significantly comparing fertilization with no fertilizer application. The results about niche-fitness and productivity showed that under RW, MW, NW and LW conditions, the means of fitness (F(t)) were 0.6631, 0.5670, 0.5174 and 0.4763 kg/hm2, and the means of grain yield were 3 159.2, 2 345.2, 1 735.7 and 1 380.6 kg/hm2, respectively. Compared RW, MW and NW with LW, the fitness increased by 39.2%, 19.0% and 8.6%, and the mean of grain yield increased by 128.8%, 69.9% and 25.7%, respectively. These results indicated that increased water supply had a significant positive impact on improving fitness and productivity. Meanwhile, fertilization also played an active part in improving fitness and productivity. Statistical analysis showed that the grain yield was linear interrelated with the fitness values. The evaluation results under 16 different treatments showed that low fertilization treatment (LF), medium fertilization treatment (MF), high fertilization treatment (HF) under RW condition, MW with HF, and NW with HF, the growth condition of crop belonged to suitable; the other 11 treatments belonged to moderate or inaptitude. The maximum value of fitness (0.8351) and the highest grain yield (4 275.8 kg/hm2) occurred consistently under rich water with high fertilization, which indicates this combined treatment measure is optimal for niche-fitness and productivity in semi-arid regions.