The main objectives of nitrogen (N) fertilization to cropland are to obtain high target yield, accepted product quality and economic benefits, and maintain or improve soil fertility. In recent years, overemphasizing the environmental effects of N fertilization by unreasonable "reducing N" are becoming fashion while paying less attention to the production target and soil fertility, which might lead to reduce crop yield or product quality, or soil fertility. Here, we define the principles of rational N fertilization, i.e. in a given climate-soil-crop system with given relatively stable agronomic managements (such as crop rotation, tillage, cultivar and irrigation etc.), N rate should be in an rational range which could achieve high target yield, accepted product quality and economic benefits, maintain or improve soil fertility, and control N losses (thus environmental effects) in an acceptable level. We discussed the methods for determining the rational N rate and the indicators for evaluating N management. We emphasize that the essential of reducing current farmers' conventional unreasonable high N rates is to minimize N losses via improving N fertilization techniques rather than simply cutting the N application rate. We also emphasize that the rational N fertilization should implement the "4R" nutrient stewardship or techniques together with the recycling of manure and straw to cropland, and balancing other nutrients include phosphorus, potassium, middle and micronutrients, and integrating to other agronomic managements. We suggest to do long-term field experiments in different climate-soil zones to study crop yield, product quality, economic benefits, ammonia volatilization, nitrate leaching, nitrous oxide emission and soil fertility simultaneously, and to explore the relationship among fertilizer N, soil N and crop N in these climate-soil -crop systems, and further to evaluate the production target, environmental effects and soil fertility comprehensively. By using the indicators of N input, N output, N surplus, N use efficiency, N losses and change of soil organic carbon (SOC). We need form rational specific fertilization techniques by machinery to minimize N losses, thus to maximize crop N uptake according to the regional climate-soil crop systems and production conditions.