Desertification is one of the most serious ecological problems in the semi-arid regions of North-East Asia. The most effective way combating desertification is vegetation rehabilitation though little is known about plant growth in desertified regions. The purpose of this study was to quantify topological and geometrical responses of shrubs to different water conditions and to develop a functional–structural plant growth model of Artemisia halodendron, a major shrub species in these regions, by using the Lindenmayer system. We measured physical units of plants growing under average semi-arid conditions, and analysed growth and assimilate allocation of plants raised under different water conditions. Finally, we developed a model to fit the recorded growth increments and allocation ratios of assimilates to each plant component under different water conditions. The outputs of the model were generally faithful to plant topological and geometric responses to water conditions. The present model’s unique points are: (1) Topology is controlled by phytomer number. (2) Geometry is defined by growth rate and allocation ratio of assimilates according to water conditions. (3) The allocation ratio of assimilates is logically estimated from LWR and shoot/root ratio.