Ultrasound and vacuum were respectively employed to enhance CO2 desorption in a water scrubbing biogas upgrading system. Results showed that incomplete CO2 desorption could cause a high CO2 content in the water and seriously affect the purity of the product gas. Vacuum had a strong enhancement effect on CO2 desorption. When a vacuum of 0.04MPa was used to enhance CO2 desorption, the amount of the stripping air could be reduced to 1/16-th of that without enhancement, indicating that vacuum could greatly enhance CO2 desorption and significantly decrease the amount of the stripping air, which was expected to reduce a large amount of energy consumption. In contrast, the enhancement effect of ultrasound was not so obvious for CO2 desorption in the desorption column with air stripping, since the solution could be well desorbed by gas stripping, though ultrasound could strongly affect the static CO2 desorption.