The calcium concentrations in soybean milk and its material were determined by flame atomic absorption spectrometry（FAAS）. The optimal dilution ratio of soybean milk and the amount of digestion acid were discussed by single-factor experiments. Effect of Lanthanum oxide solution on the determination of calcium in soybean milk was also studied. The results showed that the optimizing dilution ratio of soybean milk was 1∶3; the digestion acid consumption of soybean milk and soybean meal was 30mL and 35mL, respectively. The addition of lanthanum oxide resulted in the better sensitivity （0.136μg/ （mL·%））, lower detection limits（0.01μg/mL）, and higher recovery（98.1%）. The calcium concentration decreased 10.4% in the soybean milk processing.