The development of miniaturized instrumentations for specific analytical purposes has been one of the most important momentum in the field of instrumental innovation. Sequential injection system as a miniaturized analytical platform provides an alternative for field of analysis of heavy metal contaminants. It will exhibit more powerful capability with the introduction of atomization function and improvement of detection sensitivity. In this work, a miniature long-optical path atomic absorption spectrometric system is developed with dielectric barrier discharge (DBD) low temperature micro-plasma as atomizer. Mercury and methylmercury vapor is generated in a sequential injection system. It is directed to flow through a gas-liquid separator, a glass wool moisture-removal microcolumn and the DBD atomizer, and finally transported into the long optical-path detection cell for quantitative analysis by atomic absorption spectrometry. A 10 mA lamp current is used with a mercury hollow cathode lamp, and a 260 V negative high voltage for the photomultiplier is set. The detection sensitivity of the present system is highly improved by the increase of absorption optical path (1.1 mm i.d., 400 mm length), and the atomization of hydride is completely achieved by the DBD micro-plasma atomizer. Meanwhile, a glass wool moisture-removal microcolumn integrated in the flow system effectively eliminates the influence of concomitant moisture during the vapor generation process, which avoids the drift of absorbance baseline. The absorbance arising from mercury is recorded by turning off the DBD atomizer, while the total absorbance from both mercury and methylmercury is measured by turning on the DBD atomizer. Further experiments demonstrated satisfactory additivity for the absorbance arising from inorganic and methyl mercury, which provides basis for the determination of mercury and methylmercury. With a sampling volume of 1.0 mL, detection limits of 0.3 and 0.4 μg·L-1 are achieved respectively for inorganic and methylmercury, along with RSD values of <4%. The reliability of the present system is demonstrated by analyzing certified reference materials and real samples for mercury speciation.