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Chlorophyll (Chl) a/b ratio has been predicted to increase with the increase in irradiance but responds differently under varying nitrogen (N) availability. Since few studies have ever examined light and N stress among plants in arid ecosystems, this study attempted to understand such phenomenon in Haloxylon ammodendron (C. A. Meyer) Bunge at Gurbantunggut Desert, China, by investigating the seasonal trend in Chl a/b ratio relative to light and N content during the three-year growing seasons. The results revealed that regardless of irradiance, Chl a/b ratio increased with the progression in seasonal courses while chlorophyll concentration [Chl] decreased relative to the decrease in N content. N was negatively correlated to Chl a/b ratio and positively correlated to [Chl]. Adaptation to light/shade conditions did not affect Chl a/b ratios and hence contradicted the universal application of low Chl a/b ratio as an indicator of shade tolerance in plant species. Our results suggests that although morphological adaptations "assimilating shoots" enhances the species survival in the fragile arid ecosystem, water availability and N are probably the main drivers of biochemical plasticity. Although our results challenges the universal application of Chl a/b ratio to classify sun/shade plants under field condition, these ratios can be applied for rapid detection of plant stress in arid ecosystems, especially when combined with concomitant measurement of N.