Abstract: A new physical quantity, air kerma strength, was proposed to replace the apparent activity of the brachytherapy source to characterize the source strength, which is reported by the American Association of Physicists in Medicine (AAPM) task group No.32 report. ¹²⁵I seeds are widely used for interstitial brachytherapy treatment. It is very important to establish the primary standard to realize the traceability and transmission of the physical quantity. The existing primary standard free air chamber (FAC) can’t measure the air kerma strength of ¹²⁵I in a high accuracy, because of the characteristics of low energy and low dose rate of ¹²⁵I seeds. Therefore, a special ionization chamber was designed and manufactured for the realization of the absolute measurement of air kerma strength from a single ¹²⁵I seeds. In order to meet the basic requirement of ionization chamber design, that is charged particle balance, the structure design of key components of ionization chamber was studied by Monte Carlo and theoretical calculation, which included diaphragm, high voltage electrode, collecting electrode, potential rings, divider resistors, et al. A large conical beam irradiation was designed to increase the coverage of the measurement volume to increase the deposited energy of the secondary electrons. The distance between the high voltage electrode and the collecting electrode is 3-200 mm. The response of the ionization chamber with different thicknesses of high voltage electrode was determined by utilizing the MCNP code. The high voltage electrode was made of Al-coated polyethylene terephthalate (PET) foil. Two types of the collecting electrode of the ionization chamber were compared with the deposited energy in measurement volume. The type of the PET foil has a 0.07% effect of the deposited energy in the ionization chamber, and the type of the 1 cm PMMA sheet causes a 7.5% energy deposited change. Finally, the collecting electrode consists of a radius of 5 cm Al-coated PET foil with the width of 9 cm potential guard. The effect measurement volume varies from 235 cm³ to 1 570 cm³ and the collection volume varies from 1 847 cm³ to 12 315 cm³. Compared with the primary standard FAC, the ratio of effective measurement volume to collection volume increases from 0.3% to 12%. Charge integration method was used to measure the air kerma strength of ¹²⁵I seed. And the measurement result of the air kerma strength is 1.250 μGy·m²·h^-1（Urel=1.6%, k=2） of the nominal value of activity of 3.404×10⁷ Bq. By further optimizing the design, this extrapolation ionization chamber will be applied to realize the quantity of air kerma strength. This work provides a technical basis for value dissemination of brachytherapy.