In order to solve the problems of fruit quality decline and low nitrogen utilization rate caused by blind or excessive application of nitrogen fertilizer in watermelon production，nitrogen nutrition diagnosis and fertilizer recommendation of watermelon during the whole growth period based on soil and plant analyzer development value(SPAD value) was carried out，with a view to providing a theoretical basis for establishing a real-time diagnosis technology of nitrogen nutrition and developing a precise nitrogen nutrient management model in watermelon using SPAD instrument. A medium-late maturing watermelon cultivar ‘Jincheng 5’ was used as the test material，field experiments were conducted in Zhangye city，Gansu province from 2020 to 2021. Different nitrogen levels(0，80，160，240，320，400 kg/hm²)were designed，the SPAD value，chlorophyll contents，and nitrogen contents at different leaf orders and different detecting position of leaf were determined，and the change characteristics and their influences on yield and quality were analyzed. A real-time nitrogen management(RTNM)model based on SPAD value for nitrogen nutrition diagnosis and recommended fertilization of watermelon was established，and the productive verification of RTNM model was carried out. The tip of first fully-expanded leaf in seedling stage，the middle of third fully-expanded leaf in vines stage and the middle of function leaf in fruiting stage were more suitable for SPAD detection，which could obtain good relativity of SPAD with chlorophyll content and nitrogen content. By the regression analysis of the SPAD value of the indicator leaf and the yield，quality index and nitrogen application rate of watermelon，the SPAD threshold range for watermelon yield exceeding 60000 kg/hm² were obtained as follows:49.1-51.0 in seedling stage，60.3-64.1 in vines stage and 64.8-67.5 in fruiting stage. The SPAD threshold range for soluble solid content of watermelon exceeding 11.5% were as follows:49.0-50.5 in seedling stage，59.8-63.6 in vines stage and 64.0-66.5 in fruiting stage. The fertilizer schemes for SPAD value changes by 1 unit were as follows:30.96 kg/hm² in seedling stage，31.95 kg/hm² in vines stage and 50.51 kg/hm² in fruiting stage. Moreover，compared to the conventional application of fertilizer(N 240 kg/hm²)，the fertilizer nitrogen recovery efficiency were increased by 30.53% significantly under the condition that the amount of nitrogen fertilizer decreased by 15.29%，meanwhile，the yield and quality of watermelon were also improved under the RTNM model. In conclusion，for watermelon cultivar ‘Jincheng 5’ planted in Hexi corridor of Gansu province，the suitable indicator leaves were the tip of first fully-expanded leaf，the middle of third fully-expanded leaf and the middle of function leaf，and SPAD threshold were 49.1-50.5，60.3-63.6 and 64.8-66.5 in seedling stage，vines stage and fruiting stage，respectively，under the mode of nitrogen nutrition diagnosis and fertilization recommendation with SPAD instrument.