Nitrogen is one of the most important nutrient elements that affect the growth and external quality of flower crops. The aim of this study was to quantitatively investigate the impacts of nitrogen on external quality of cut lily, so as to optimize the nitrogen management for cut lily production in greenhouses. For this purpose, six experiments with different planting dates and levels of nitrogen treatment were conducted in greenhouses in Lianyungang (34°42' N, 119°30' E) and Nanjing (32°N, 118°E) from August 2008 to May 2011. The cultivar used in the experiments was Lilium spp.‘Sorbonne’ and‘Siberia’. Four levels of available nitrogen treatment (25 mg (N)/kg (soil), 45 mg (N)/kg (soil), 65 mg (N)/kg (soil) and 85 mg (N)/kg (soil)) were used in the nitrogen treatment experiments. The plot of an area of 0.8 m² for each nitrogen treatment with three replicas was arranged in a randomized block design. In all experiments, 3 plants of each plot (9 plants per nitrogen treatment) were randomly selected for non-destructive measurements once every 4 days after planting. The non-destructive measurements include plant height, leaf number, length of first flower bud and diameter of first flower bud. Statistical information on the relative yield of different rank was also collected at harvest stage. Photosynthetically active radiation (PAR) and air temperature above the canopy inside the greenhouse were monitored automatically every 10 s, and 30 min averaged data were recorded using a datalogger (CR1000; Campbell Scientific Inc.). The integrated photo-thermal index, PTI, was used to describe the changes of the external quality traits (leaf area index, plant height, leaf number, length of first flower bud, diameter of first flower bud) and relative yield. Effects of the accumulated leaf nitrogen content (dry weight based) at flower bud showing stage on the dynamics of the external quality traits and the relative yield of different rank were quantified based on the experimental data. Based on these quantitative relationships, a model for predicting the effects of nitrogen on external quality of cut lily in greenhouse was developed. Independent experimental data were used to validate the model. The results showed that our model gave satisfactory predictions of external quality traits and the relative yield of different rank for cut lily grown in greenhouse. The coefficient of determination (r²) and the relative root mean squared error (rRMSE) between the predicted and measured value of the cut lily were, respectively, 0.92 and 0.09 for leaf area index, 0.85 and 0.10 for plant height, 0.86 and 0.14 for leaf number, 0.91 and 0.13 for length of first flower bud, 0.88 and 0.12 for diameter of first flower bud, 0.88 and 0.18 for relative yield of rank one, 0.88 and 0.15 for relative yield of rank two, 0.91 and 0.18 for relative yield of rank three, and 0.91 and 0.21 for relative yield of rank four. The model developed in this study can be used for the optimization of nitrogen management for cut lily production in greenhouses, and provides reference approaches for the optimization of nitrogen management for cut lily production in open field in the Yangtze River delta.