To obtain the optimized structure of photonic crystals with dispersion-free slow light, based on the traditional line-defect structure of photonic crystal waveguides, we introduce a novel type of slow light photonic crystal waveguide with a line defect which is surrounded by four rows of quadrate air holes, and optimize the structural parameters of the two rows of quadrate air holes. By means of the plane wave expansion (PWE) and 2D finite difference time domain (FDTD), our numerical analysis shows that, at the center wavelength of 1550 nm, our new design can present a wideband dispersion-free slow light, which in possession of a wavelength bandwidth of 8.34 nm, an average group index of 61 and a group velosity dispersion (GVD) of 104.