Based on coupled waveguide with eye-shaped scatterer cavities with high-quality Q values, operations such as changing the parameters of the scatterers and changing those of the coupling cavities reveal the patterns about the variation of slow light effect on photonic crystal coupled waveguide and can achieve very high group index within the flat band. Simulations prove that regardless of the number of coupling cavities, slow light with decent flat band and structures with ultra-slow light and group index between 4.10×104 and 1.35×105 can be achieved by adjusting the ratio of the major axis to the minor axis and by shifting the scatterers.