The invention provides a method of calculating the amount of slag discharged by a slurry shield machine using density meters. The method comprises the steps that firstly a slurry feed density meter and a slurry discharge density meter are additionally arranged above the middle segments of a slurry feed pipeline and a slurry discharge pipeline in the second layer of an equipment bridge separately, the data signal lines of the slurry feed density meter and the slurry discharge density meter are connected with a control system of the slurry shield machine, and the slurry feed density meter and the slurry discharge density meter can display the slurry feed density value and the slurry discharge density value through the control system in real time. In the process of one-ring tunneling, the cumulative total feed amount J m<3> can be displayed automatically in the control system through a slurry feed flow meter, and the cumulative total discharge amount P m<3> slurry can be displayed automatically in the control system through a slurry discharge flow meter, the average slurry feed density value M (average feed) kg /m<3> can be converted automatically in the control system through the slurry feed density meter, the average slurry discharge density value M (average discharge) kg /m<3> can be converted automatically in the control system through the slurry discharge density meter, and finally, the actual slag amount G' of one-ring is obtained automatically through the control system to determine whether the tunneling requirement is satisfied.