Abstract: Geological samples were digested by the mixed acid system of HNO₃-HClO₄-HF. The Tl was adsorbed by polyurethane foam and determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Under the optimized operation conditions for ICP-MS, the effective monitoring and correction for the long-term drift of the instrument signal, and reduction of the interference of the matrix effect were obtained by using Re as an internal standard. The linear range was 0-2 μg/mL, the detection limit was 0.003 μg/g, and precision RSD (n=11) was 2.50%-4.98%. The method was validated by national standards, proving to be accurate and reliable.