In the phase shift modulation type optical components constructed by traditional dielectric waveguides with the low refractive index constant based on the total inner reflection mechanism, the length of the modulation region is usually millimeter or centimeter order of magnitude, while their horizontal size is micrometer of order of magnitude, therefore, the most typical characteristics of optical waveguide devices are long and narrow. Reducing the size of optical waveguide devices is a hard problem in the development of highly dense integrated optical circuits. The emergence of photonic crystals provides a new approach to develop highly dense integrated optical circuits. The plane wave expansion method is used to calculate the dispersion curves of the line-defect type photonic crystal waveguides. It is observed that there is a large change of wave prorogation constant near the guided-wave band edge, corresponding to a little change of refractive index of the material. If the work frequency is selected near the guided-wave band edge, the phase shift modulation length is expected to largely reduce. The finite-difference time-domain method is used to demonstrate the results above. Calculated results indicate that there is strong enhancement effect of refractive index phase shift modulation near guided-wave band edge: for the refractive index change of 0.46%, the phase shift modulation length in these waveguides is only 11.7% of that in conventional uniform dielectric material. This enhancement effect is originated from the special flat dispersion properties near the guide-wave band edge, and it is expected to be applied to high dense photonic integrated circuits after further research.