Based on the rigorous coupled-wave theory,the merit functions for the diffraction bandwidth and variance from center wavelength are proposed to analyze the diffraction property of broadband metal multi-layer dielectric grating(MMDG).When the groove depth,residual thickness,duty cycle and incident angle are 290 nm,10 nm,0.28 and 60° respectively,the optimized MMDG exhibits an ultra-broadband top-hat diffraction spectrum with average efficiency exceeding 97% over the 190 nm wavelength range centered at 1 053 nm with TE mode.Numerical simulation shows that the optimized MMDG has excellent fabrication tolerance.The optimized broadband MMDG should be useful for chirped pulse compression.