Abstract: In order to address the problem of reduced charge collection efficiency of existing commercial ionization chambers under high pulse dose conditions due to severe ion recombination effects, the ion transport process under high pulse dose conditions was studied in depth based on the numerical solution of the one-dimensional transport model, and the key parameters affecting the collection efficiency of the ionization chamber were systematically analyzed. The design and key dimensions of the flat-plate ionization chamber were determined through finite element analysis to improve its performance under ultra-high dose rate environments. The simulation results show that with the increase of the electrode spacing, the average movement path of ions before being collected is extended, resulting in a significant increase in the collection time; the lengthening of the average movement path increases the probability of free electron attachment, and the free electron fraction decreases significantly, resulting in a decrease in the probability of being collected by the electrode; at the same time, the recombination probability of positive and negative ions increases, and the electric field distribution is distorted, which ultimately leads to a significant decrease in the charge collection efficiency. Under the condition of a single pulse dose that meets the requirements of flash radiotherapy, the charge collection efficiency of the ionization chamber with a smaller electrode spacing (such as 0.25 mm) remains high. In the single pulse dose range of 0.18-2.18 Gy, the ionization chamber response of the ionization chamber prototype is well linear with the EBT4 readout value.