Surface plasmon polaritions microcavities have attracted considerable attention due to their high quality factor, ultra-small mode volume and wide applications in optoelectronic devices. A kind of surface plasmon polaritions metal-coated microdisk resonator is theoretically simulated and optimized by using finite element method. The quality factor and mode volume of the plasmonic mode of the microdisk are theoretically investigated by considering different parameters of the microdisk such as the bottom radius, the thickness of dielectric, and the thickness of metal coating. High quality factor (>1000), ultra-small mode volume surface plasmon polaritions microcavities are achieved for the optical telecommunication wavelength around 1550 nm. Finally, a refractive index sensing application of the optimized microdisk is also investigated, which achieves a high sentivity of 300 nm/RIU.