Abstract: As a high-sensitivity radiation detector, ionization chambers play an important role in radiotherapy dose calibration and radiation environment monitoring. However, the field of radiotherapy dose measurement is highly dependent on imported equipment, and there is a lack of systematic experimental data supporting the dose stability and reliability of domestic detectors. This study focuses on the domestic QPPC40 parallel plate ionization chamber, quantitatively measuring its key dosimetric parameters in medical accelerator beams. These parameters include energy response, ion recombination, polarity effect, repeatability, leakage current, and dose linearity. The goal is to enhance the dosimetric performance of domestic detectors and provide practical recommendations for clinical applications. The measurement results indicate that the energy response correction factor k_Q for 6 ～ 25 MV photon beam relative to Co-60 γ ray ranges from 0.991 to 0.961, with a repeatability of 0.02%. For 4 ～ 22 MeV electron beams, the k_Q factor ranges from 0.953 to 0.887. At an operating voltage of -400 V, the ion recombination correction values for dose rates of 0.04-5.13 Gy/min range from 1.002 to 1.008. The polarization correction factor is 1.001, and the leakage current is within ±0.05%. The tested indicators generally meet the technical requirements of the JJG 912-2010 verification regulation for therapeutic level ionization chambers. However, the calibration coefficients and energy response consistency of ionization chambers from the same batch show some variability. It is recommended to conduct interim inspections during use and to monitor the long-term stability of the water-absorbed dose calibration coefficient and energy response of the detector.