Abstract: The active calcium was extracted from pulverized oyster shells by lactic acid prepared as ultrafine calcium carbonate by carbonization method under the regulation of trypsin. The features of ultrafine calcium carbonate were characterized by scanning electron microscopy (SEM) and infrared spectroscopy (IR). Through single-factor test and orthogonal experiment, the process conditions of lactic acid extracting active calcium from oyster shells were studied. The results showed that the calcium extraction rate of oyster shells could reach 39.46% under 1∶20 of the solid-liquid ratio system (oyster shell mass∶lactic acid solution volume), 2 mol/L of lactic acid concentration, 50 ℃ of the extraction temperature, 30 min of the extraction time and 350 r/min of the stirring rate. The effects of trypsin addition on the preparation of ultrafine calcium carbonate by the carbonation of the active calcium extracted from oyster shells by lactic acid were investigated. SEM, IR and other characterization results showed that the vaterite-type nano calcium carbonate agglomerated into 300-400 nm ultrafine microspheres with uniform size, could be prepared when the crystallization of calcium carbonate was controlled by adding trypsin at the ratio of 3∶1. The research is of great significance for the development and utilization of oyster shells and the preparation of new food-grade calcium preparations.