We proposed a method for ultra-high group index slow light with high optical buffering performance based on photonic crystal waveguide coupled with a cavity. By introducing cavity analogous to rhombus shape in photonic crystal waveguide center, slow light properties and buffering performance are studied. Numerical results through plane-wave expansion method and model field distribution through finite-difference time domain method show that both the rhombus cavity radii and some discriminatory rods around rhombus cavity have a plentiful effect on slow light properties and buffering fulfillment. By adjusting the cavity rod radii, we obtained high group index of 5163 with buffering bit length L<sub>bit</sub> about 17.968 μm and delay time T<sub>s</sub> reaching to 51.967 ps through the waveguide-cavity length of 3.02 μm. Moreover, low group velocity dispersion can be achieved, with governable positive and negative values. To regulate the optimal parameters to increase the group index and demonstrate the delay time performance, some discriminatory rods around rhombus cavity are adjusted in the upper and lower edges confronting each other in the waveguide, and ultra-high group index as exceedingly large as 22350 is obtained that is extremely higher than previous studies. Simultaneously, the corresponding buffer bit length Lbit and delay time Ts reach respectively to 25.8279 μm and 225.7783 ps through the waveguide-cavity length of 3.0306 μm. We designed a simple structure but a more generalized that may contribute a requisite theoretical basis for potential industrial applications in the storage capacity properties of high group index for optical buffering and optical communication systems.