With the higher and higher requirements of micro-electro-mechanical-systems(MEMS) inertial sensors for micro-displacement detection precision, micro-displacement detection technology has become a research hotspot in the field of MEMS inertial sensing. The emergence of sub-wavelength gratings provides a new development prospect for micro-displacement measurement. In this paper, the off surface double-layer sub-wavelength grating displacement sensing system is studied. The displacement sensing characteristics of the system of equal period and unequal period are studied. The relationship between the transmittance of diffracted light and the off surface double-layer displacement of the dual sub-wavelength gratings and the influences of the key structural parameters of the system on the sensitivity of the displacement detection are simulated by Rsoft software. The structure optimization and performances of the two dual grating micro-displacement sensing systems are simulated and compared. The high sensitivity advantage of sub-wavelength grating displacement detection is verified in theory.