Based on the finite difference-time algorithm, the factors influencing second harmonic generation in photonic crystals consisting of centro-symmetric materials are analyzed. The results show that photonic band gap is the key factor inducing the generation of second harmonic (SH). The suitable angle of incidence of pumping wave, waveguide length and fill factor will be helpful for the enhancement of SH. For the structure in this paper, when the angle of incidence 20 degrees-30 degrees, SH intensity will be five times to that when the angle is nearly 90 degrees. According to the results of our computations, the critical wave-guide length is 50 mu m for the maximal intensity of SHG.