The invention relates to ultrahigh pressure measurement, calibration and source tracing, and belongs to the field of detection. The measurement comparison method for an ultrahigh-pressure piston pressure gauge provided by the invention is characterized by comprising an ultrahigh-pressure piston pressure gauge to be measured and a standard ultrahigh-pressure piston pressure gauge, wherein both the ultrahigh-pressure piston pressure gauge to be measured and the standard ultrahigh-pressure piston pressure gauge are respectively connected to a manganese-copper pressure gauge and a cut-off valve, the manganese-copper pressure gauge connected with the ultrahigh-pressure piston pressure gauge to be measured is connected with the other manganese-copper pressure gauge connected with the standard ultrahigh-pressure piston pressure gauge, the two ultrahigh-pressure piston pressure gauges are enabled to reach pressure balance through assistance of the two connected manganese-copper pressure gauges and the two cut-off valves, and small pressure difference between the two ultrahigh-pressure piston pressure gauges is compared through measuring differential pressure variations of the two manganese-copper pressure gauges in the balancing process. The measurement comparison method provided by the invention realizes pressure difference measurement under high pressure, thereby realizing unification between measurement values reproduced by ultrahigh pressure and normal pressure national standards, and ensuring precision pressure measurement under an ultrahigh-pressure state.