In operation of orifice flowmeter, measurement error will gradually get large. In order to identify factors affecting the measuring accuracy, selected is SST k-w turbulence model to conduct numerical simulation analysis on flow field changes upstream and downstream of orifice plate in orifice flowmeter. SST turbulence control equation is more suitable for flow field calculation in convection reduced pressure zone. Determined is the optimum pressure tapping point through simulation experiment and conducted is qualitative analysis on the effect of pressure tapping point location on the measuring accuracy. Under experimental conditions, temperature difference upstream and downstream of orifice plate throttling may result in error of 0.067 52%0 in measurement accuracy. Sharp degree drop of hole edge at opening rectangular entrance will lead to great changes of flow field upstream and downstream of orifice plate and pressure differential drop upstream and downstream of throttling, consequently, it leads to less measurement values than real ones. Therefore, sharp degree of orifice plate shall be test after orifice flowmeter operates for a period of time, and orifice plate is replaced if necessary so as to avoid affecting the measuring accuracy.