We report the enhancement of the bandwidth and the response flatness of wavelength conversion through cascaded second-order nonlinear processes by sinusoidally chirped optical superlattices (SCOSs) in lithium niobate crystal. The conversion bandwidth of the SCOS is significantly larger than that of uniform and segmented gratings. The SCOS shows very flat conversion response in the 1.5-m region. The physical origin of the bandwidth enhancement is ascribed to the plentiful reciprocal wave vectors of the SCOS, which provide enough momenta to simultaneously satisfy quasi-phase matching conditions for both second-harmonic generation and difference-frequency generation.