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Pitot tube is widely used for fluid velocity and flow rate measurement in industrial fields. However, improper installation of the Pitot tube is actually inevitable, which causes additional errors to the measurement. In this paper, through exploring the relationship between the pressure sensing tube and the fluid flowing direction, a correlation was deduced for calculating the theoretical flow rate of the Pitot tube under improper installations, and the influence of different deflection angles on Pitot tube measurement performance was discussed. Based on the experiments carried out in a DN200 air horn, the theoretical flow rates were compared with the measured. The methodology of correcting the flow rate of a Pitot tube by the correction factor was put forward, and the correction factors of different mounting deflection angles were calibrated. The results show that the correction factor is basically symmetrical with the horizontal deflection angle, whereas it decreases first and increases afterwards when the vertical deflection angle varies from -30deg to 30deg. By adopting the correction factor, the flow measurement accuracy of the Pitot tube under improper installations can be improved significantly.