Objective:To improve the dosimetric accuracy of cell irradiation experiments by developing a method of accurately measuring the absorbed dose of ultra-thin solution in culture dishes under 200 kV medium-energy X-rays using EBT3 films.Methods:EBT3 film dose calibration was performed under Cyberknife 6 MV beam, and the beam quality (half-value layer) and effective energy of the 200 kV beam used in this study generated from Small Animal Radiation Research Platform through measurements and calculations were obtained to determine the EBT3 energy response correction factor. The 200 kV beam was utilized to irradiate three commonly used culture dishes filled with ultra-thin liquid placed on EBT3 films and the corrected EBT3 doses were taken as the liquid absorbed doses. The dose linearity of immersed films was also measured and analyzed. In addition, after modeling the irradiation environment, the independent Monte Carlo calculations of the liquid absorbed dose were performed by MCNP5 program. The calculation results were compared with the film measurement results to verify the accuracy of the measured doses.Results:The 200 kV beam had a half-value layer of 8.77 mm aluminum and effective energy of 57.4 keV, corresponding to an energy response correction factor of 0.889. The average liquid absorbed doses of large, medium and small culture dishes measured by EBT3 films under the specified parameters of 200 kV beam were (1.434±0.004) Gy, (1.467±0.011) Gy and (1.469±0.027) Gy after correction, respectively. The percentage errors from the corresponding Monte Carlo calculation doses were 0.07%, -0.70%, and 0.47%, respectively, where the relatively consistent results could be found. In addition, the dose linearity of immersed EBT3 films was also good, with coefficient of determination R2=0.9972. Conclusion:The method of measuring the dose of ultra-thin cell solution using EBT3 films proposed in this study is feasible, and the dose results obtained yield high accuracy under 200 kV beam.