On the basis of field experiments conducted by authors for a few years, the research results published by many others were collected and analyzed. It was showed that the water effect and fertilizer effect on crop production were related to experiment method. The result obtained from field experiments was opposite to that from simulated experiments because the concept of “water” in the experiments was not the same. The result of rainproof-shed experiments or pot experiments didn’t coincide with the field situation. In many cases, nutrient shortage was still the limit factor to increase crop yield in arid zones in Northern China. The effect of irrigation water on crop production depended on the soil water content before sowing, precipitation during crop growing, soil fertility and irrigation time, etc. At the present water resources situation, the main way to increase crop yield in most farmland of arid zone in Northern China was increasing nutrients supply. Enhancing the storage of rainfall during summer fallow period and raising soil water content before sowing were important for winter wheat production.