A new method based on molecular mass for rapid determination of inorganic arsenic in algae by electrospray ionization mass spectrometry was developed. Several conditions affecting accuracy and sensitivity such as extraction method, pH of solvent, reductant, chelating agent, extractant and instrument parameters were studied. Under the optimum conditions, the calibration curve of As(Ⅲ) was linear in the range of 0-151.4 ng/mL, the calibration curve of As(Ⅴ) was linear in the range of 0-175.0 ng/mL, and the correlation coefficients were all above 0.99. The limit of detection(S/N > 3)was 0.024 mg/kg and the limit of quantitation (S/N > 10)was 0.048 mg/kg. The recoveries at different spiked concentration levels ranged from 81.0% to 95.6% with relative standard deviations(2.52%-7.54%). This method is simple, of high sensitivity and good reproducibility, without interference of organic arsenic. It can be applied to analyze inorganic arsenic in algae and can provide technological support for food safety management.