Mid-infrared supercontinuum generation in dispersion-engineered ZnSe rib waveguides was investigated for the first time. Numerical results showed that the zero-dispersion wavelength can be shifted to a shorter wavelength by adjusting structural parameters and refractive index contrast between the core and cladding layers in the waveguide. The optical field can be well confined in the 4- and 8-μm wide waveguides with a 2-μm thick cladding layer of Ge₅As₁₀S₈₅ glass. The effect of waveguide parameters on the bandwidth of the supercontinuum spectrum at a 5-cm-long waveguide was also simulated to understand the effect of the pump wavelength and structure parameters on the supercontinuum generation. Our results showed that supercontinuum output could vary over a wide range depending on structural parameters of the waveguide， the pump power and wavelength. An ultrabroad supercontinuum spectrum from 3.0 up to 12.2 μm （> 2 octaves） was confirmed in a 4 μm-width waveguide with a peak pump power of 20 kW and a pump wavelength of 4.5 μm， which is promising as one of the on-chip supercontinuum light sources for many applications such as biomedical imaging， and environmental and industrial sensing in the mid-infrared.