The invention discloses a purification system for biogas, which comprises a biogas pressurizing device, an accumulator tank, a water scrubber, a drier, a resolving tower, an air blower and a water storage tank, wherein the biogas enters the water scrubber after being pressurized, water is sprayed downwards from the upper part of the water scrubber body in at least two grades to remove carbon dioxide and hydrogen sulfide in the biogas; purified biogas is discharged from the top of the water scrubber, and dried by the drier for standby use; the water which is rich in carbon dioxide and hydrogen sulfide flows through the resolving tower, blown by gas in the resolving tower, and used in the water scrubber again to remove carbon dioxide and hydrogen sulfide after carbon dioxide and hydrogen sulfide in the water are removed. The purification system is formed according to the scheme that the solvabilities of carbon dioxide, hydrogen sulfide and methane are different in water, and the water scrubber is built according to a physical adsorption process; no filler nor chemosorbent is needed; the purpose of biogas purification is realized; the structure is simple; the energy consumption as well as the cost is low; the environment is protected; the produced biogas is qualified; water resource is saved; emission of three wastes is avoided.