Abstract: Brachytherapy is an effective treatment for cancer. ¹²⁵I seeds are widely used for the low dose rate brachytherapy, which has the advantages of short half-life, easy to make microstructure and easy protection. Medical physicists make treatment plans mainly based on the formula of the dose distribution of particle sources around tissues. The quantity characterizing the intensity of seeds has changed from the quantity of apparent activity to the air kerma strength. In order to measure the intensity of the source accurately, it has become a hot issue in international research. This paper introduces the development of dose distribution formula of ¹²⁵I seeds in tissue. The status of absolute measurement of particle source intensity, such as air-kerma strength, reference air-kerma rate and absorbed dose to water is analyzed. And the primary standard and measurement method of brachytherapy seeds are summarized. In our country, the quantity of ¹²⁵I brachytherapy seeds used for the treatment increases. The relevant domestic standards was imperfection. And there is a lack of measurement technology and the methods of the air kerma strength of ¹²⁵I seeds. In view of the current situation in China, some suggestions on the brachytherapy seeds intensity measurement are put forward to realize the source intensity transmission and traceability in China.