The invention discloses a pressurizer oil returning gas fleeing amount measuring method which comprises a step 1, namely returning oil of a pressurizer is first guided into an oil-gas separator for gas collecting; a step 2, namely a gas flow meter connected with the oil-gas separator is used for measuring the oil returning gas fleeing amount collected by the oil-gas separator and storing the measuring results in a recorder; a step 3, namely the pressurizer returning oil in the oil-gas separator is guided back to an engine oil bottom shell. According to the method, the pressurizer returning oil first flows to the oil-gas separator for gas collecting, the gas flow meter connected with the oil-gas separator is used for measuring the gas fleeing amount in the oil-gas separator and storing the measuring results in a recorder, then the returning oil flows back into the engine oil bottom shell, engine oil is used for separating gas above the oil-gas separator and gas above the oil bottom shell, gas in an engine crank box is prevented from entering the oil-gas separator, gas in the oil-gas separator is prevented from entering the engine crank box, measuring is accurate, and operation is simple.