A system for purifying mother liquid steam of polytetrafluoroethylene dispersion liquid comprises a concentration kettle (1), a defoaming pot (2), a gas-liquid separator (3) and a falling film absorption tower (4). A steam exit of the concentration kettle (1) is connected to an entrance of the defoaming pot (2). An exit of the defoaming pot (2) is connected to a feeding port of the gas-liquid separator (3). A gas exit of the gas-liquid separator (3) is connected to the falling film absorption tower (4). A liquid exit of the gas-liquid separator (3) is connected to the concentration kettle (1). A method for purifying mother liquid steam of polytetrafluoroethylene dispersion liquid by using the system is further provided.