A method for diagnosing a fault situation of a negative-pressure generation line using a fuel tank pressure sensor that detects the pressure of a fuel tank is provided. The method detects pressure in a fuel tank measured by a fuel tank pressure sensor when a turbocharger is operated for boosting and a purge valve is operated and diagnoses a current situation as a fault situation in which an engine negative-pressure generation line remains open when a pressure change value calculated based on the pressure difference of the fuel tank exceeds a reference value. A warning of a fault situation is then output when a fault is determined.