An effective approach is proposed to broaden the spectrum of high-efficiency second harmonic generation in a one-dimensional photonic crystal based on the cascaded structure. By controlling the thickness of the joint layer, it is possible to realize a flat-top or quasiflat-top impurity band centered at the fundamental wavelength due to mode splitting effect in coupled cavities. Simulation results reveal that the spectrum of generated second harmonic exhibits a hump-like or multi-peak profile with wavelength tuning. It is a salient feature that the spectral stability of efficiency enhancement could be greatly improved compared to the conventional Lorentzian profile while maintaining an ultrahigh Q factor. Such merit of spectra re-shaping can significantly relax the stringent requirements for ultrahigh-Q microcavities in practical applications, e.g. fabrication inaccuracy, thermal variation, and wavelength detuning.