The underreamed anchor is widely used in geotechnical engineering because of its advantages of large tensile capacity and high support efficiency. And the grouting length and the type of casing tube for free anchor length will affect the stress and deformation characteristics of anchor. Therefore, field experiments of underreamed anchor under different working conditions were carried out, including fixed anchor length grouting + PE corrugated tube (design scheme), full-length grouting + PE casing tube (common scheme in actual construction), fixed anchor length grouting + HDPE casing tube, full-length grouting + HDPE casing tube. The load-displacement (Q-s) curve, load-elastic displacement (Q-s_e) curve, load-plastic displacement (Q-s_p) curve, elongation ratio of free section and stiffness coefficient of cable under different working conditions were compared and analyzed. The test results show that compared with PE corrugated tube anchor, HDPE casing tube anchor could protect and isolate the free anchor length and ensure the deformation performance of the free anchor length under the same grouting length. When the same casing tube is used, the increase of grouting length will lead to the decrease of free anchor length elongation value, but will increase the stiffness coefficient of the underreamed anchor. On the basis of the experimental study, it is suggested that the scheme of free anchor length grouting + HDPE casing tube or full-length grouting + HDPE casing tube should be adopted in the field construction of anchor cable, so as to ensure that the stress and deformation performance of underreamed anchor meet the design expectation.