Abstract: The static expansion vacuum standard is a primary vacuum standard. Its measurement precision mainly depends on the precision of volume ratio measurement. The pressure of gas before and after expansion were measured by a higher-pressure piston gauge and a lower-pressure digital piston gauge respectively, but the pressure after expansion was measured through a CDG （1.33kPa FS） as the comparator. By the above method, the volume ratios were measured and the combined standard uncertainties were analyzed.