A device and a method for diagnosing a positive crankcase ventilation (PCV) breather line. The device includes: a crankcase pressure sensor that is installed on the PCV breather line including a front end connected to an intake line and a rear end connected to a crankcase and detects a pressure inside the crankcase; and a processor that diagnoses an abnormality of the PCV breather line based on the pressure of the crankcase.