Abstract: Water is the important factor affecting the growth and external quality of chrysanthemum. The aim of this study was to quantitatively investigate the effects of water on the external quality indices (plant height, stem diameter, leaf number, diameter of flower head, length of flower neck) of standard cut chrysanthemum grown in a solar greenhouse. Experiments of standard cut chrysanthemum (Chrysanthemum morifolium ‘Shenma’) with different planting dates and different levels of water treatment were conducted in a solar greenhouse in Beijing from August 2006 to June 2007. The physiological product of thermal effectiveness and PAR(PTEP) was used to describe the seasonal change of the external quality indices. Effects of substrate water potential on the dynamics of plant height, stem diameter, leaf number, diameter of flower head, length of flower neck were quantified based on the experimental data. These quantitative relationships were then integrated into a model to predict the effects of substrate water potential on the external quality indices of standard cut chrysanthemum in solar greenhouse. Independent experimental data were used to validate the model. The results showed that the coefficient of determination (R2) between the simulated and measured values of plant height, stem diameter, leaf number, diameter of flower head, length of flower neck based on the 1:1 line were 0.98, 0.93, 0.98, 0.93, 0.86, respectively, and the relative prediction error (RE) between the simulated and measured values were 8.79%, 5.66%, 8.45%, 12.2%, 12.9%, respectively, for plant height, stem diameter, leaf number, diameter of flower head and length of flower neck. The model developed in this study gives satisfactory predictions of the external quality indices of the standard cut chrysanthemum, hence, it can be used for optimizing water management for standard cut chrysanthemum production in solar greenhouses.