We report difference-frequency generation (DFG)-based wavelength conversion with sinusoidally chirped optical superlattices (SCOSs) in lithium niobate waveguides. The SCOS shows much broader bandwidth, larger pump-wavelength tolerance and flatter response with some conversion efficiency penalty than the periodic grating with the same length. Especially, the conversion bandwidth of the SCOS is over the whole conventional band and long-wavelength band, which is very helpful to broadband optical communications. The main advantages of the DFG-based wavelength conversion with SCOS structures over the cascaded second-order effect-based wavelength conversion are higher conversion efficiency, broader bandwidth, better pump-wavelength tolerance and response flatness when the device is shorter. In order to ensure the conversion quality, analysis shows that the manufacture tolerance of the SCOS period should not more than 0.02lm.