In order to detect sources of arsenic in the Spirulina platensis power from the ordos alkali lake in Inner Mongolia, Arsenic content of culturing materials, nutrient solution and algal power in the period of initial cultivation and normal cultivation for Spirulina platensis was studied. The results showed that the water for culturing Spirulina platensis and NaHCO3 providing the arsenic into the nutrient solution in the period of no culturing for Spirulina platensis, the arsenic content correspondingly keeps in a stable state not only in the period of initial cultivation and normal cultivation and long terms of cultivation, the arsenic content would cut down by cleaning the algal power auxiliarily with distilled water.