In this letter, a pure geometric phase based all-dielectric metalens is designed and aimed to enhance the tolerance of imaging in the longitudinal direction. Numerical simulation for the designed metasurface is carried out using the pure geometric phase spin decoupling design method combined with the finite difference time domain (FDTD) method. The calculated results show that designed metasurface enables the characteristics of controllable polarization and extended focal length, achieving a focal depth of 8 mm along the propagation direction, compared with 4.5 mm for a conventional metalens. Furthermore, we also design and numerically validate a terahertz metalens that can generate two focal points with extended focal length, and the polarization states of the two extended focal points are orthogonal to each other, demonstrating the multiplexing functionality of the designed metalens. The unique and effective approach may open a new avenue for tomography and information encryption.