Radiation and temperature are the major environmental factors affecting flowering date and external quality traits of flower crops. The aim of this study is to develop a model for predicting the flowering date and external quality traits of cut tulip grown in solar greenhouses for the purpose of optimizing radiation and temperature conditions inside solar greenhouses through adjusting planting date and planting density. Three experiments with different planting dates and densities were conducted in a solar greenhouse located at Lianyungang, Jiangsu (34°42' N, 119°30' E) during 2007 and 2008. Two tulip cultivars (Tulip gesneriana cvs 'World Favorite’ and 'Golden Parade’) were used in the experiments. During each experiment, photosynthetically active radiation (PAR) and air temperature above the canopy inside the solar greenhouse were automatically monitored, and crop data of development and external quality traits were collected for model development and validation. After planting, 4 development stages (sprouting, leaf unfolding, flower bud visible and harvesting) were observed. After sprouting stage, 5 plants of each plot (15 plants per density treatment) were randomly selected for non-destructive measurements once every 3 days. The non-destructive measurements include plant height, number of unfolding leaf per plant, length of basilar stem, diameter of basilar stem, length of flower neck, diameter of flower neck, length of flower bud, diameter of flower bud, length and width of individual leaf. Statistical information on the relative yield of different rank was also collected at harvest. The integrated effects of PAR and temperature (depending on planting date and density) on the development and external quality traits of cut tulip crops were quantified according to the experimental data. Based on these quantitative relationships, a photo-thermal model was developed to predict the flowering date and external quality traits of cut tulip grown in the solar greenhouse. Independent experimental data were used to validate the model. The results showed that our model gave satisfactory predictions of flowering date and external quality traits of cut tulip crops. The determination coefficient (R²) between the predicted and observed development stages was 0.95, and the root mean squared error (RMSE) between the predicted and observed days of sprouting, leaf unfolding, flower bud visible and harvesting were 0.7d, 1.3d, 2.9d and 1d, respectively. The R² and RMSE between the predicted and the measured values of external quality traits were, respectively, 0.97 and 30.8mm for plant height, 0.98 and 0.2 for number of unfolding leaf per plant, 0.98 and 3.5mm for basilar stem length, 0.98 and 0.1mm for diameter of basilar stem, 0.98 and 5.5mm for length of flower neck, 0.97 and 0.1mm for diameter of flower neck, 0.98 and 1.2mm for bud length, 0.97 and 0.4mm for bud diameter, 0.95 and 0.8% for relative yield of rank A, 0.97 and 0.3% for relative yield of rank B, and 0.96 and 0.9% for relative yield of rank C. The model developed in this study may be used for optimizing planting date and density for cut tulip production in solar greenhouses.