Abstract: In order to develop detection skill on benzene series, proficiency testing of benzene series in surface water organized by an international agency was attended. The samples were pretreated by purge and trap, and the benzene, toluene, p-xylene and m-xylene, o-xylene concentrations in the surface water determined by Gas Chromatography and Mass Spectrometry. The components of uncertainty were analyzed and calculated by establishing a mathematical model, and the uncertainty in the process of test was assessed. The result was finally submitted to the organizer. The statistics showed that the satisfactory rate of all the benzene series was 100%. Firstly the uncertainty of benzene series in surface water was evaluated from aspects of reference solution, internal solution, multifunctional automatic sampler, repeatability of measurement and recovery ratio, respectively. Then each component of uncertainty was combined, and the expanded uncertainty of benzene, toluene, p-xylene and m-xylene, o-xylene was obtained, being 0.077, 0.095, 0.250 and 0.140, respectively, when k was equal to 2.