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By analyzing the gas expansion process from the upstream chamber via an orifice and a very rapid-opening ultra-high vacuum(UHV)gate valve to downstream one,the standard pressure analytical model based on the dynamic vacuum calibration apparatus in millisecond range developed by Lanzhou Institute of Physics(LIP)is deduced theoretically and corrected by real gas characteristics and temperature changes.According to the Knudsen criterion,there is no free molecular flow regime in the area in front of the orifice during the gas expansion,so the chocked flow approximation is adopted to reduce the difficulty of numerical computation.Under this approximation and the full opening of the rapid valve,the standard pressure expression is calculated theoretically,and the upstream chamber pressure and temperature changes are obtained by numerical simulation during the gas expansion from 100 kPa to 10 kPa.Also,experiments are performed using capacitance diaphragm gauges(model CDG045Dhs).The uncertainties between the measured pressure and the simulated and theoretical ones are 10％and 4.65％,respectively,which indicates that the apparatus can generate predictable pressure changes in the millisecond range and the conceptual model can better approximate the calibration results.Finally,the orifice conductance,and the correction factors of real gas characteristics and temperature change to the standard pressure are calculated according to the simulation results;the corrected standard pressure expression is obtained.