Although the effect of elevated carbon dioxide (CO2) on Phalaenopsis plant flowering, biomass, and photosynthesis has received intensive study, whether elevated CO2 affects plant requirements and sensitivity to potassium sulfate (SOP) during the reproductive growth stage remains unclear. To evaluate the combined effect of CO2 and SOP provision on crassulacean acid metabolism orchids, we cultivated Phalaenopsis Queen Beer ‘Mantefon’ under ambient and elevated CO2 treatments (≈ 400 or ≈ 720 μmol×mol-1, respectively) and four levels of SOP supply for 20 weeks after treatments (WAT): potassium and sulfate levels by 10.41 and 1.96 mmol·L-1 (SOP1), 5.98 and 0.90 mmol·L-1 (SOP2), 12.80 and 1.96 mmol·L-1 (SOP3), and 14.83 and 3.16 mmol·L-1 (SOP4), respectively. The number of floral buds and flowers decreased in the plants grown under elevated CO2 than in those grown under ambient CO2, regardless of the SOP level; however, the reduced production of floral buds and flowers did not affect the dry mass of shoot, root, and spike at 20 WAT. There were significant interactive effects of CO2 and SOP on root biomass accumulation and net CO2 uptake. The stimulation of biomass partitioning on the root, as a sink source, observed due to the uptake of elevated CO2 was improved under increased SOP supply. Under ambient CO2, the leaf critical SOP level was SOP1 for root and spike biomass accumulation. Plants grown under elevated CO2 were more sensitive to SOP treatments, with higher essential leaf levels of SOP.