The invention discloses a method and device for extracting soil dissolved nitrogen. The method comprises the following steps: A, collecting soil samples, namely recording morphological characteristic information such as soil colors, structures, textures, consistence and new growth bodies according to soil genetic horizon, placing the samples into a closed plastic bag, putting into a refrigerator, and keeping the samples in a dark place; B, culturing soil, namely bringing cutting ring samples back to a laboratory, developing the measurement of field water capacity values of the soil, placing the samples into a constant temperature and humidity incubator, and culturing in the dark place; C, centrifuging the samples, namely taking out the soil samples, completely transferring the samples to an acrylic cloth bag, sealing the bag opening, putting into a polypropylene centrifuge bottle, tightly screwing a bottle cover, and treating in an ultra-high-speed refrigeration centrifuge to obtain a soil solution; D, sub-packaging and storing the solution; and E, performing analytic determination, wherein a polypropylene bottle cover and a polypropylene solution collecting box are connected with a polypropylene bottle body by virtue of screw threads, and a filtration separation plate is connected with the polypropylene bottle body. The method disclosed by the invention is feasible, is simple and convenient to operate, wide in applicability, stable and reliable in measured data results, high in accuracy and good in repeatability, and ensures that the composition and content of different soil dissolved nitrogen can be accurately measured.