The invention discloses an automatic condensed water draining device for a positive and negative pressure system of an air conditioner. The device comprises a three-way body and a sealing cover, wherein the three-way body is provided with a longitudinal pipe; a first side pipe and a second side pipe are correspondingly arranged at two sides of the bottom part of the longitudinal pipe; the side edge of the top part of the first side plate is positioned in a central axis of the second side pipe, namely, the first die pipe is lower than the second side pipe; a sealing floating ball is arranged atan opening in the inner side of the first side pipe in the longitudinal pipe; a wastewater draining port is formed in the sidewall of the bottom part of the longitudinal pipe, and a wastewater draining plug is also equipped and is sleeved with an O-shaped ring; outer screw threads are formed in the opening ends of the longitudinal pipe and the second side pipe; and the sealing cover is provided with inner screw threads and a sealing ring and is connected to the opening end of the longitudinal pipe or the second side pipe. Compared with the prior art, the device has the advantages that the condensed water draining function of a positive pressure system can be met based on different mounting methods, and the negative pressure condensed water draining function is also met; and the device iscompact in structure, and simple to mount.