The influence of the air hole size of different rings and the doped dielectric cylinders size in the cladding on the band gap is investigated by plane-wave expansion method(PWM), for photonic crystal fibers(PCF) based on the triangular periodic structure, the composite honeycomb structure and the improved composite honeycomb structure. It is demonstrated that photonic crystal fiber band gap is most sensitive to the air hole size variation in the first two rings. When the change of the air hole size in the m-th ring is constant, the effect of the air hole size changing on the band gap decreases with the increasing of m. Through improving the arrangement of these dielectric cylinders in structure or doping the high refractive-index medium, the band gap can be regulated or made wider effectively. Compared with other two structures, photonic crystal fiber with improved composite honeycomb structure can achieve a wider band gap.