The invention discloses a shield tail brush sealing performance testing device and a testing method thereof. The shield tail brush sealing performance testing device comprises a framework, a pressure testing groove, a top cover and a pressure monitoring device. A plurality of tested tail brushes are installed on an in-groove bottom of the pressure testing groove. The internal of each tested tail brush is connected to an oil pipeline having an oil supply through in a sealing manner, wherein the oil pipeline is arranged in an oil supply through hole on the bottom of the pressure testing groove, and thus is communicated with an oil supply system. A rightmost cavity in an inclination direction of the tested tail brush passes through a slurry pipeline in a slurry supply hole formed on the pressure testing groove bottom in a sealing manner and is communicated with a slurry injection system. A pressure signal input terminal of the pressure monitoring device is connected to pressure testing devices which are respectively arranged in a high pressure cavity and a low pressure cavity through a cable. The shield tail brush sealing performance testing device and the testing method thereof can adopt the mode of the shield tail brush and arrange the shield tail brushes under different pressure conditions, and avoid tunnel construction risks caused by leakage due to inappropriate mode selection of the tail brush.