Abstract: The mass of the special small weights used in liquid-medium piston gauges is calculated with reference to the effective area of the zero-pressure piston, while the mass of the large weights only considers the deformation of the piston system caused by loading the large weights. As a result, the actual pressure measured by the piston gauge loaded with nominal pressure weights is less than the nominal pressure. Based on the basic principles of piston pressure gauges, a measurement value correction model for liquid-medium piston gauges with a measurement upper limit of 25 MPa and above is derived. The results of the relative pressure correction values for piston gauges with upper measurement limits of 60 MPa and 250 MPa show that the absolute value of the relative pressure correction value increases nearly linearly with the number of small weights loaded but exhibits a nonlinear increase with a gradually weakening trend as the number of large weights loaded increases. The limit value of the relative pressure correction value is proportional to the pressure deformation coefficient and the upper measurement limit of the piston gauge. Overall, the influence of small weights on the relative pressure correction value is greater than that of large weights, and the relative pressure correction value reaches an extreme value at the pressure measurement point with the largest number of small weights loaded near the upper limit of the piston measurement. The relative pressure correction limit of the 250 MPa piston is −5.17×10⁻⁵, exceeding the maximum allowable error of the 0.005-level piston gauge. Based on the limitation of the maximum tolerance of the special weight mass, the output measurement values of 25 MPa piston gauges of grade 0.01 and above and 250 MPa piston gauges of grade 0.02 and above must be corrected to ensure the accuracy of their pressure value transmission.