The invention discloses a device for measuring the mass of a stock solution in a rectangular soft stock solution package and a calculation method. The device comprises pressure sensors and a single-chip microcomputer which is used for receiving and processing signals of the pressure sensors. The multiple pressure sensors are ranked at intervals in an array mode. The single-chip microcomputer receives the signals of the pressure sensors and then output the mass of the stock solution through a display screen after calculation. By the adoption of the device for measuring the mass of the stock solution in the rectangular soft stock solution package, the mass of the stock solution in the rectangular soft stock solution package can be rapidly and accurately measured outdoors, the contact area of the measured soft stock solution package and the ground and the pressure change condition can be reflected in real time by the pressure sensors which are ranked in the array mode, values of the pressure sensors of which values are changed are calculated through the single-chip microcomputer, and then the mass of the solution in the soft stock solution package to be measured is acquired in real time, the whole measurement process is high in speed, and manual intervention is not needed. By the adoption of the calculation method, the mass of the stock solution in the soft stock solution package to be measured can be accurately acquired.