According to the differences in physical properties between CO2 and CH4, the temperature and pressure of bubble point and dew point of the biogas with different volume fractions could be obtained with PR equation. Based on Aspen HYSYS, the liquefaction process of separating CO2 from biogas was designed. The effects of low temperature liquid separation on CO2 and CH4 from biogas were analyzed. The experiment of biogas purification was carried out and the result was consistent with that of simulation. The results of the experiment showed that the volume fraction of CH4 increased from 58.2% to 82.8%, the volume fraction of CO2 decreased from 36.4% to 8.8%, and the volume fraction of N2+O2 was less than 4.0%. 