A photonic-crystal coupled waveguide is proposed by introducing two line-defects to perfect triangular lattice. By analyzing the dependence of band shape on various structure parameteral, unique band with approximate zero group velocity can be obtained in an optimized photonic-crystal coupled waveguide. Moreover, by chirping the widths of the photonic-crystal waveguides in the optimized structure, a perfect wideband slow light is obtained, it possesses the bandwidth of 13.24 nm and the average group index of 28, which is verified by means of two-dimensional finite-difference time-domain (FDTD) simulations, and our numerical analysis shows that Gaussian pulses passing through the coupled waveguide have a relative temporal spreading below 10%.