Reducing CO2 content will significantly improve biogas quality. In this study, a CO2/Ca(OH)2 precipitation reaction was used to purify biogas and prepare CaCO3 nanoparticles simultaneously in a membrane reactor. Results showed that the rate of biogas purification was greater than 99%, and CaCO3 particles were successfully formed in the membrane reactor, which was detected with an average particle size between 300 and 550 nm with chemical additives. Further experiments revealed that the flow rate of biogas, temperature, and different chemical additives showed negligible effects on the biogas purification efficiency, while CaCO3 particle size decreased with the increasing flow rate and increased with the increasing temperature. The average particle sizes of the CaCO3 particles were 334, 537, 437, and 867 nm, corresponding to the addition of EDTA, ZnSO4, Na6O18P6, and the control, respectively. This method was proven to be an effective technique of simultaneously purifying biogas and synthesizing nano-CaCO3.