The invention discloses a multifunctional well completion pipe string sealing performance detecting device and an application method and belongs to the well completion pipe string sealing performance detecting technology. The defect of a float collar and a float shoe in the prior art is overcome. The multifunctional well completion pipe string sealing performance detecting device comprises an upper connector, a main body, a piston cutting sleeve, a fluid discharge hole, an upper O-shaped sealing ring, a lower O-shaped sealing ring, a constant pressure pin and a lower connector. The upper connector is sleeved with the upper end of the main body, and the lower end of the main body is connected with the lower connector. The piston cutting sleeve is fixed to the main body with the constant pressure pin 7 and sealed through the upper O-shaped sealing ring and the lower O-shaped sealing ring. The multifunctional well completion pipe string sealing performance detecting device has the significance that the air tightness of a sleeve is verified first, the sleeve is pressurized and opened after the sleeve is qualified, and then well cementing construction is carried out, so that the air tightness of the well completion sleeve is guaranteed effectively, and reworking construction and scrapping of an well are avoided.