Surface plasmon excited by metallic grating integrated on metal-semiconductor-metal can greatly improve the absorption of devices. In order to deeply explore the excitation and resonant discipline of surface plasmon, a design of metal-semiconductor-metal based on ultra-thin germanium is proposed. By using finite difference time domain (FDTD) method, the effects of grating period, grating depth, grating space, and thickness of the active layer on the performance of surface plasmon resonance supported device are investigated in detail. The structure parameters of the device are optimized, and the mechanism of surface plasmon excited by each interface as well as spectrum absorption enhanced by surface plasmon resonance is analyzed in detail. Simulation results show that the germanium device with an ultra-thin active layer of 400 nm has a high absorption in the communication band, especially at the wavelength of 1550 nm the normalized spectral absorption can be as high as 53.77% with an enhancement factor of 7.22. Surface plasmon resonance can greatly improve the optical response of high-speed optoelectronic device, thus an efficient way is provided to solve the trade-off between photodetector responsivity and speed of the device.