We report difference frequency generation-based wavelength conversion in step-segmented quasi-phase matched (QPM) gratings. It is shown that both the pump bandwidth and the signal bandwidth are efficiently widened by adding the segment number of QPM grating and optimizing the poling period of the first segment and the period shifts between adjacent sections. Especially, flattop conversion efficiency response with peak-to-peak ripples less than 0.2 dB, which is very helpful to multichannel conversion in WDM systems, can be achieved simultaneously. For 3-cm-long waveguide, the signal conversion bandwidth and the pump bandwidth reach 166 nm and 0.88nm, respectively, meanwhile the maximum efficiency is-0.55dB.