An analytical method for the determination of ultra-trace impurities in high purity molybdenum was established by inductively coupled plasma tandem mass spectrometry (ICP-MS/MS). In MS/MS mode, H2 was used as the reaction gas to measure Si and Ca through the H2 on-mass method. O2 was employed as the reaction gas to measure the Cd via the O2 on-mass method, and P, As, Se, Ta, Sn, Sb, Ba, and W were measured by the O2 mass-shift method. NH3/He was adopted as the reaction gas, so that Na, Mg, Al, K, and V could be measured by NH3/He on-mass method, and Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn could be measured by the NH3/He mass- shift method. Pb, Bi, Th, and U were measured in single quadropole (SQ) mode without gas. Compared with conventional collision/reaction cell (CRC) inductively coupled plasma mass spectrometry (ICP-MS), the background equivalent concentration (BEC) and detection limit of analyte in this method were lower, and the mass spectrum interference could be eliminated more thoroughly. Under optimized working conditions, the calibration curves of analytes showed good linearity with linear correlation coefficient (R2)≥0.999 7. The detection limit was 0.04～50.1 ng·L-1, the spike recoveries were 92.2%～107.4% and the relative standard deviation (RSD) was ≤4.3%, indicating that the proposal method has excellent accuracy and precision. The results of the sample analysis showed that the method can be used for the determination of 28 impurity elements in high purity molybdenum powder with purity of 5N (≥99.999%).