Flattop broadband wavelength conversions based on difference frequency generation were fulfilled by utilizing step-segmented quasi-phase matched gratings.For the same waveguide length,high conversion efficiency,flat response and broad signal bandwidth can be obtained simultaneously by adding the segment number of quasi-phase matched grating and optimizing the poling period of the first segment and the period shifts between adjacent sections.Analysis of the structure parameters on conversion efficiency,signal bandwidth and response flatness is performed as well.The conversion bandwidth in a 3-cm-long waveguide reaches 166 nm,which is over the whole conventional band and long-wavelength band,meanwhile the maximum efficiency is -0.55 dB and the peak-to-peak ripples less than 0.2 dB.In addition,the comparison among step-segmented gratings,sinusoidally chirped optical superlattices and segmented gratings is carried out,the step-segmented gratings shows higher conversion efficiency and flatter response than the other two gratings,and has much broader bandwidth than sinusoidally chirped optical superlattices.