As the uncertainty component introduced by the effect of air density measurement is one of the main compositions of the measurement result uncertainty of the piston pressure gauge, the paper analyzed the effect degree with formula derivation to PG7607 primary piston pressure gauge, the results show that: the effect of the air density to the measurement results of the piston pressure gauge is related to the environmental atmospheric temperature, humidity, pressure status at the using site; related to the measurement result uncertainty of environmental atmospheric temperature, humidity, pressure at using sites. With the laboratory condition, the effect of the air density to measurement results of the piston pressure gauge is in the range from 0.25 ppm to 0.35 ppm in China, the effect of air density to the measurement results of piston pressure gauge is in the range from 0.31 ppm to 0.33 ppm in Beijing. The analysis method in this paper can be applied to other type piston pressure gauges, besides the final computation result differences due to the difference of temperature, humidity and pressure measurement errors.