The focus of this paper is the sub-wavelength imaging of gradient index photonic crymstals. The gradient index photonic crystals is a parallel plate composed of silicon and circular air holes. The gradient change of photon crystal refractive index is realized by adjusting the structure of each column of pores. The focusing process of the gradient index photonic crystals is simulated by finite-different time-domain (FDTD) algorithm. The results showed that proper modification of the optical path difference could greatly optimize the focusing effect. Furthermore, changing the focal length of photonic crystals and the structure of the central air hole could also optimize the focusing effect. By combining the above three elements, the gradient index photonic crystal was finally designed which could realize sub-wavelength focusing. The half-width of the focused spot was 0.3447 λ , which was located at 1.45 λ outside the photonic crystal. To improve application performance, a dynamic focal length adjustment system was designed on the gradient index photonic crystals. The focal length range of 1.1374 λ～2.6264 λ could be adjusted without changing the structure of gradient index photonic crystals. Meanwhile, the half-width of the focal spot was less than 0.4 λ.