In photonic crystals (PCs) waveguides, the speed of light can be significantly reduced due to the weak coupling between adjacent high-quality-factor nanocavity resonators. However, little research focuses on the length between two cavities associated with slow light characteristics. A waveguide based on a PC within a square array with adjustable rods spreading regularly at the edge and center of the cavity is proposed. Slow light properties in this new structure are studied by using the plane wave expansion method. Through a series of experiments, we find the most suitable coupling length to compromise various parameters of slow light, which is just the half-period of that in a conventional structure. Furthermore, the numerical results reveal that the group velocity is below 3.532×10−3 c in the new model. The corresponding normalized delay-bandwidth product can reach as high as 0.3068 since the group velocity dispersion is at the range of (−5 to 5)×106. All of above signify that the very small size and ultraslow light, as well as outstanding dispersion performance, are realized and there are some advanced features, such as low loss and high integration, in this half-periodic structure.