Separation of carrier-free iodine from low-level iodine samples and accurate measurement of ultra-low-level I-129 in microgram iodine target are essential but a bottleneck in geology and environment research using naturally produced I-129. This article presents a detection technique of accelerator mass spectrometry (AMS) for accurate determination of ultra-low-level I-129 in carrier-free AgI-AgCl sputter targets. Copper instead of aluminum was selected as the suitable sample holder material to avoid the reaction of AgI-AgCl powder with aluminum. Niobium powder was selected as thermally and electrically conductive matrix to be mixed with AgI-AgCl powder, in order to obtain and maintain a stable and high iodine ion current intensity, as well as less memory effect and low background level of I-129. The most optimal ratio of the Nb matrix to the AgI-AgCl powder was found to be 5:1 by mass. The typical current of I-127(5+) using AgI-AgCl targets with iodine content from 5 to 80 mu g was measured to be 5 to 100 nA. Four-year AMS measurements of the I-129/I-127 ratios in standards of low iodine content and the machine blanks showed a good repeatability and stability.