The invention relates to a soil auger for an undisturbed bisect soil column of filed soil, which comprises a power device, an undisturbed soil inner cylinder, a helical outer cylinder and soil cutting devices. The power device is positioned on the top of the soil auger; the power device, the undisturbed soil inner cylinder and the helical outer cylinder are arranged concentrically up and down. A fixing sleeve for the undisturbed soil inner cylinder on a fixed shaft on the lower part of a direction-changing gear case of the power device is coupled to the undisturbed soil inner cylinder. The undisturbed soil inner cylinder is arranged in the helical outer cylinder, and downwards passes through a shield bearing on the lower part of an inner wall of the helical outer cylinder; and two parallel sheet helixes are welded on the outer wall of helical outer cylinder; two helical soil scrapers are welded at the bottoms of the two parallel sheet helixes; and a set of soil cutting device is arranged at the opposite positions of the outer wall of helical outer cylinder respectively. The soil auger has the advantages of simple structure, simple and convenient operation and convenient carrying, is suitable for field survey, observation and sampling related to soil, guarantees the undisturbed performance and the representativeness of the soil, reduces the working capacity, avoids damaging the soil sample environment, and has the advantages of wide application, accuracy and high efficiency.