We design the THz lens made of slit-groove-based metamaterials with tunable far-field focal length as well as subwavelength resolution, based on surface plasmons(SP) diffraction theory into spoof SP of THz region. In THz regime, the curved depth profile of grooves from both sides of metal slit produce directional beaming and mimic SP at the same time. By arranging the depth of grooves in traced profile, it is possible to optimize the focal position in THz region without changing the size of structure. It is performed numerical simulation of a designed structure through finitedifference time-domain (FDTD) method and shows the subwavelength imaging of the designed position. In addition, the change of focal length and the relative Ex phase are observed in the simulation and help to comprehend a subwavelength 1D slit-groove-based metamaterials in THz regime.