Flow around a 2-D cylinder pressure probe placed in uniform flow, free jet flow, and wind tunnel flow was analyzed with potential flow theory and simulated with numerical method. Blockage effect was investigated under several typical flow Mach numbers. The result from numerical simulation shows a similar trend to the one from potential flow method while varies in quantity. Wind tunnel walls accelerate the flow near the probe and thus produce a blockage effect;Boundary of free jet flow, however, decelerates the flow and thus produces a “negative” blockage effect. A maximum incoming Mach number exists when the probe is calibrated in wind tunnel in high subsonic condition due to choking caused by shocks and shock induced separation. The critical Mach number varies with blockage ratio, which makes high Mach number impossible to achieve in large blockage ratio condition. The blockage effect itself is unavoidable for calibration or measurement although a sufficiently small blockage ratio brings minor effect. Correction can be implemented based on the numerical simulation result presented in this paper and further works.