Effects of structural parameters of photonic crystal fiber (PCF) on its eigenmode field distribution are analyzed with the full vector finite element method (FEM). The numerical calculation results show that light field will be confined in fiber core when the PCF structure is with multi-layer air hole and fiber core, long distance between air holes and large filling ratio of air. High order mode appears when the number of fiber core layers, distance between air holes and filling ratio of air increase. In addition, the increasing of the number of fiber core layers and distance between air holes compensates the power leaking at fiber core arising from the decreasing of filling ratio of air. It is demonstrated that the single-mode transmission with large mode field is feasible when we decrease filling ratio of air and increase the number of fiber core layers and distance between air holes. The core radius and effective mode area can reach 40 μm and 3717 μm2 respectively while the power concentration of fiber core is 68.32% with four-layer air hole, two-layer fiber core, air filling ratio of 0.01 and distance between air holes of 20 μm.