<p>The ionization chamber produces significant space-charge and ion recombination effects at ultra-high dose rates, posing a challenge for dose monitoring. In addition, there is no generally accepted ion correction model for dosimetry in FLASH radiotherapy, making it crucial to monitor the dose at ultra-high dose rates accurately and in real time. In this study, the air pressure of the ionization chamber was reduced to perform real-time beam monitoring, and a Faraday cup was used for calibration for active dosimetry. To study the saturation effect of the ionization chamber, the drift, attachment, recombination, and diffusion processes of the electron-ion pairs were modeled using finite-element analysis based on physical phenomenological principles, and the correction factor was calculated. The experimental results showed that the FLASH ionization chamber measures good dose linearity at a dose rate of approximately 0.2 Gy/s. When the air pressure of the chamber was adjusted to 10 mbar, the response of the FLASH ionization chamber was linear at a dose rate of approximately 50 Gy/s, with the residuals within 2%. Furthermore, by using physical phenomenology to resolve the process of electron-ion pair motion in the sensitive volume of the ionization chamber, the analytical model better describes the saturation effect of carbon ions at ultra-high dose rates. The maximum deviation in the calculated correction factor is less than 10%. We studied the saturation effect in dose measurement, achieving accurate and fast dose and profile position measurement across different dose rates in a wide range based on the Heavy Ion Research Facility in Lanzhou.</p>