An extremely simple, cost-effective, and one-step method was developed for the preparation of solid-phase microextraction (SPME) fibers via the in situ synthesis of carbonaceous nanoparticles on the surface of stainless steel fiber from a candle flame. The prepared SPME fiber provided excellent adsorption capability toward Hg2+ derived with sodium tetraethylborate and was explored for the separation, preconcentration, and long-term preservation of ultratrace mercury in natural water samples. Moreover, the SPME fiber was further utilized for the highly sensitive routine analysis and field analysis of mercury with a commercial atomic fluorescence spectrometer (AFS) and a miniature point discharge optical emission spectrometer (μPD-OES), respectively. Under the optimum conditions, detection limits of 0.0005 μg L-1 and 0.007 μg L-1 together with relative standard deviations (RSDs) less than 5.8% were obtained for Hg2+ by headspace SPME-AFS and SPME-μPD-OES, respectively. The practicality of the prepared SPME fiber was validated via the determination of Hg2+ in real water samples with satisfactory recoveries (79-115%). Long-term preservation of mercury at parts per trillion level was undertaken at -20 °C and the sample loss rates were less than 5% after 9 days of storage, respectively, indicating that the storage performances of mercury provided by the proposed method are much better than that obtained by conventional methods. Notably, the extraction performance of the SPME fiber is not decreased obviously even after more than 100 times of operation. Due to its advantages of high sensitivity, high stability, simple operation, low cost, and low energy consumption, this method provides an avenue for the field analysis and long-term preservation of mercury in the field of environmental analytical chemistry.