The generation and control mechanisms of a broadband supercontinuum (SC) in photonic crystal fibers (PCF) with twozero dispersion wavelengths are demonstrated by simulating the evolution process of a femtosecond pulse numerically. The results reveal that the intrinsic higherorder dispersions and nonlinear effects of PCFs cause the pump pulse, whose center wavelength locates between the anomalous dispersion region, decay into fundamental solitons and two dispersive waves simultaneously. In virtue to the enhancement of the two dispersive waves by their phasematched solitons, SC will be formed by the two dispersive waves, on which oscillations happen for the interference. Then, the limited effect on the SC applied by the two dispersive waves is analyzed quantitatively by comparing the SCs in three PCFs with different geometrical parameters. Meanwhile, the effects of the geometrical parameters are demonstrated. Based on the above conclusions, considering the analysis on the relationships between the center wavelengths of the dispersive waves and the dispersion curves of PCFs as well as their geometrical parameters, a method to control the range of SC is promoted based on the design of the geometrical parameters. As an example, a flat SC extending to the visible region is achieved by optimizing the geometrical parameter in theory.