Spirulina platensis was batch cultivated in a novel 3.5 l magnetic air-lift photobioreactor with external-loop on which 0–0.55 T electromagnetic field (EMF) was imposed to investigate the EMF effects on the algal growth and nutrition composition. At the same time, the correlative mechanism was discussed mainly based on C, N, P uptake. It was found that, EMF has double-side effects on S. Platens is cultivation depend on EMF intensity. 0.25 T EMF stress was found to be suitable for the algal cultivation enhancement increasing the maximum cell dry weight by 22% in a time period two days less than that of the control at 35 C and 2521mol photons m2 S1. In addition, the nutritional composition of S. platens is was improved in both essential amino acids such as histidine and trace elements Ni, Sr, Cu, Mg, Fe, Mn, Ca, Co and V. It was suggested it was possible to optimize the cultivation process of S. platens is by imposing suitable weak EMF which may enhance the nutrition assimilation e.g. C, N, P and minerals by S. platens is.