  We propose and design an ultra-compact and broadband tunable multimode
interference (MMI) splitter in mid-infrared based on graphene plasmonic
waveguides. The size of the device is only 0.56{\mu}m*1.2{\mu}m, which
corresponds to device area of only about 0.014{\lambda}^2, where {\lambda} is
the vacuum wavelength. And the center wavelength of the device can be tuned in
a broad band from 7{\mu}m to 9{\mu}m with the Fermi level of graphene varied
from 0.5eV to 1eV. Furthermore, the device is easy to be fabricated on chip.
