In this paper, the structure optimization design of MEMS "sandwich" inclinometer is mainly carried out. The sensing element adopts double beam-mass block structure, in which the movable mass block is connected with the surrounding frame through the double beam. The movable mass block, the top and the bottom plate constitute differential capacitance. When the external acceleration is loaded, the gap between the movable mass block and top-bottom plate will change which results in the changes of differential capacitance. The relationship between key performances and structure parameters is obtained by building up three-dimensional micromachining model of the sensing element based on bulk-micromachining process. The results from ANSYS FEA show that the sensitivity and the first-order resonant frequency are 0.76 pF/g and 576.68 Hz respectively with the linearity 0.8‰ and 3‰ for ±30° and ±90° measurement range, all of which meet the design requirements. In the end, the fabrication process of sensing element is also presented.