We present a method for quickly setting up the complex structure model of photonic crystal fiber (PCF) through directly reading the cross-section of photonic crystal fiber (PCF) by MATLAB, and numerically analyzing the band-gap and the modal field distribution in band-gap PCF (PBG-PCF) by combining plane wave method (PWM) with finite difference frequency domain (FDFD) method. Firstly, the frequency and modal effective index in PBG-PCF can be calculated by using PWM. Then, the modal field distribution and others properties of the PBG-PCF can be obtained in certain wavelength and index ranges by using the calculation results above and FDFD method. To validate its applicability, a type of commercial PBG-PCF was discussed and the influences of the structure parameters (crystal lattice structure, "atom" filling fraction, background material index and "atom" index) on the band-gap were systemically analyzed. This analytic method and calculation results may be useful for the design and preparation of the PCF with large band-gap.