It is important to measure the rare long-life radioisotopes such as ⁴⁰K and ¹²⁹I etc. in neutrino detector material. A new method based on Accelerator Mass Spectrometry (AMS) is being developed for measurement of these trace elements. At present, the main nuclides to be measured include ⁴⁰K in CsI and Liquid Scintillators (LS), and ¹²⁹I in CsI. For the measurement of ⁴⁰K, a series of ⁴⁰K standard of samples are being made and chemical procedures explored for LS sample to make the samples suitable for AMS measurement. ¹²⁹I measurement is based on the established procedures, which have a detection limit of about 10^-13 for ¹²⁹I/¹²⁷I. The background of ⁴⁰K in ion source has been checked by using ultra-pure copper target, the result shows a detection limit of <10^-15g ⁴⁰K/g may be achieved.