The invention discloses a system and a method for purifying tail gas of medical waste incineration. The method is characterized in that a spray gun is used for quickly cooling tail gas at 400-600 DEG C to below 200 DEG C in a tail gas quenching tower; the cooled tail gas enters into a semi-dry type reactor; lime and activated carbon are added in front of the semi-dry type reactor, so as to de-acidify the tail gas and absorb dioxin and heavy metal; dust is removed from the de-acidified tail gas by a bag-type dust collector; the de-dusted tail gas is subjected to spraying alkali wash through a spraying layer in a spraying absorbing tower; liquid drop carried by the tail gas after being subjected to the spraying alkali wash is removed by a demister in the spraying absorbing tower; and then the tail gas is heated after entering into a steam-tail gas heat exchanger, and lastly the tail gas is discharged from a chimney by a draught fan. By using the system and method provided by the invention, the purifying efficiency for the tail gas of medical waste incineration can be obviously increased. Especially, when a large amount of organic chloride is burnt, such as waste plastics (for example, PVC (polyvinyl chloride), PVDc (polyvinyl dichloride)), rubbers, leathers, and the like, concentration of HCL (hydrogen chloride) generated by being burnt is higher, the wet alkali wash is automatically performed, and the deacidification demand is still met, thereby reducing environmental pollution, generating no waste water and solving the problem of generating waste water by performing wet treatment.