A metal-insulator-metal (MIM) waveguide with L-shaped resonator is designed. The transmission properties of this resonator are numerically investigated by finite element method. The results show that the transmission properties of the L- shaped resonator depend on the length and the height of it. L- shaped cavity has better quality factor and better filtering effect compared with the horizontal rectangular cavity with the same size as L- shaped cavity. With the increase of the size of L- shaped cavity, spectrum exhibits a red shift. Filling the medium of different refractive index in L- shaped cavity, the MIM waveguide with L- shaped resonator allows a much more sensitive detection of small refractive index changes of the filled media inside Lshaped nano-cavity. It is easy to detect the refractive index of medium by detecting resonant wavelength in the transmission spectra, which reveals a potential sensors of the MIM waveguide with L-shaped nano-cavity.