Abstract: It is important to improve the AMS 14C dating ability for small samples, especially for some precious archaeological samples or geological samples with small contentof carbon. A serles of improvements have been done in the recent two years. The difficultpoint for small samples is that some samples contain more sulphur but less carbon. Afterimproving the sample preparation process,the efficiency of removing sulphur and synthetizing graphite rises obviously. We redesigned the sample change mechanical structure, which provides more reliable samle change operation, rebuilt the injection controlsystem by using an IPC, replaced the data acquisition system,and optimized the transmission efficiency of the whole line. A batch of small samples have been dated in the pasttwo years as shown in table 2. Now the PKU AMS has the ability to measure sampleswhich is less than 100μg.