By numerically solving the generalized nonlinear Schrdinger equation, mid-infrared supercontinuum generation in two cascaded fibers has been analyzed. The simulation results show that the spectrum has been greatly broadened in the high nonlinear SF57 photonic crystal fiber(PCF) with two zero dispersion wavelenghs by self-phase modulation, stimulated Raman scattering and dispersive waves amplification. Spectrally smooth, highly broadband mid-infrared supercontinuum in a short piece of SF57-PCF is presented here. The spectral range extends from 1800 to 3500 nm with the 3 dB spectral power density. It is found that the long wavelength edge of supercontinuum is further broadened with the increase of the peak pump power.