This study selected Dendrobium offi cinale protocorms as the experimental material and used diff erent light-quality treatmentsand calcium signalling inhibitors to explore the mechanism of the biological response of calcium to light quality. The results indicate that a 1:3 red:blue (R:B) light ratio promotes the accumulation of biomass and polysaccharides in theprotocorm. Nevertheless, this eff ect can be entirely counteracted by calmodulin (CaM) inhibitors and calcium ion-chelatingagents. The possible mechanisms of this eff ect are as follows: (1) The Ca 2+ -CaM signal-regulated photosynthetic apparatusis improved by the R:B 1:3 light, increasing the CaM content and Ca 2+ -ATPase activity. Thus, R:B 1:3 light increases thedevelopment of the photosynthetic apparatus in the D. offi cinale protocorm, enhances its photosynthetic productivity, andpromotes the activities of sucrose phosphatase; accordingly, biomass and sucrose accumulation increase. (2) R:B 1:3 lightenhances Ca 2+ -CaM signalling and regulates glycometabolic processes to promote the activities of sucrose synthase, thusincreasing fructose and glucose levels and providing suffi cient synthetic precursors to promote polysaccharide production.