Abstract: SPAD- 502 meter provided a simple, fast and nondestructive method to monitor cotton nitrogen status and predicted the need for fertilizer-N topdressing on time, but its precision was influenced by many factors. Because SPAD- 502 meter diagnosed the nitrogen nutrient status indirectly by means of measuring color of leaves (value of SPAD), it was important to understand factors that affected SPAD readings in order to improve its application effect. In this paper, the dynamic rules of chlorophyll contents of upper leaves of cotton plants were set forth and factors which influenced SPAD readings, such as cotton variety, development stage, the leaf position, the sample number of leaves and different nitrogen and phosphorus fertilizer rates were studied. The research progress on improving the precision of cotton nitrogen diagnosis by quantifying the relationship of SPAD reading of nitrogen content and the degree of aging, as well as nitrogen recommendation by quantifying the relationship of SPAD reading and topdressing were also reviewed. For a purpose of further improving the precision of nitrogen status diagnosis by SPAD-502 meter and fertilizer recommendation, a few questions need to be settled were put forward at last.