Based on the multi-pole method, a kind of step-structure photonic crystal fiber (PCF) is put forward. It has achieved flattened and ultra-flattened dispersion within the wave band of 1.1～1.8 μm by altering three structural parameters of the inner four rings (the diameter of the two inner rings holes, the diameter of the two outer rings holes, and the pitch), the absolute value of dispersion coefficient merely changing from 0.05～2 ps/(km·nm). Moreover, the effective mode-field area also has been stimulated, which is only 1/30 of that of conventional PCFs, showing that the PCFs with flattened and ultra-flattened dispersion do well in confining the light field compared with conventional PCFs. With numerical calculation and theoretic analysis, the design basis of this kind of step-structure PCF to achieve flattend and ultra-flattend dispersion within the wave band of 1.1～1.8 μm is concluded.