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Sheath expansion is a distinctive feature of the double flush mounted probe because of the embedded configuration. Previously, the sheath expansion effect was usually neglected in weakly ionized plasma dominated by collisions between charged particles and neutrals. In this work, we investigated the sheath expansion effect of the double flush mounted probe in weakly ionized plasma. Results indicate that measurements using the double flush mounted probe were also influenced to a certain extent by the sheath expansion effect in weakly ionized plasma. To eliminate the influence, an empirical analytical formula has been presented to eliminate the influence of sheath expansion. In addition, a fitting curve is given based on experimental data, which indicates that sheath expansion should be considered in processing the measured data when the plasma pressure is lower than 200 Pa. In summary, this work indicates that the ion–neutral collision is a crucial factor that affects sheath expansion in addition to the radius parameter and probes' bias, which can be extended to double flush mounted probe diagnostics in collisional plasma such as the reentry plasma sheath and high-powered plasma thruster.