<p>An in-plane high-g microelectromechanical systems (MEMS) accelerometer is designed for ultra-large acceleration measurements. The proposed device is a piezoresistive MEMS accelerometer, in which piezoresistors are employed as signal transduction elements. Unlike conventional cantilever-beam-supported structures, the proof mass is supported by four suspended thin plates to enhance structural stiffness and reliability under high-g shock loading. The sensing structure is bonded to the substrate using bonding technology. The accelerometer adopts an in-plane sensing configuration, where the applied acceleration is perpendicular to the surface of the proof mass. This configuration effectively avoids large shear stresses at the bonding interface under extreme acceleration, thereby improving the mechanical robustness and service life of the device in high-g applications. Four piezoresistors are symmetrically fabricated at the roots of the suspended thin plates to convert structural deformation into resistance variations. The feasibility of the proposed design is validated through theoretical analysis and finite element method (FEM) simulations, and an optimal structural design is obtained. Simulation results indicate that the measurement range of the accelerometer can reach 100,000 g, while the overload resistance can reach 200,000 g. The optimized dimensions include a proof mass side length of 1000<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\mu m\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>μ</mi> <mi>m</mi> </mrow> </math></EquationSource> </InlineEquation>, suspended thin plates with a length of 600<InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\mu m\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>μ</mi> <mi>m</mi> </mrow> </math></EquationSource> </InlineEquation> and a width of 320<InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\mu m\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>μ</mi> <mi>m</mi> </mrow> </math></EquationSource> </InlineEquation>, a structural thickness of 80<InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\mu m\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>μ</mi> <mi>m</mi> </mrow> </math></EquationSource> </InlineEquation>, and a gap of 5<InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\mu m\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>μ</mi> <mi>m</mi> </mrow> </math></EquationSource> </InlineEquation>between the lower surface of the structure and the substrate. The results demonstrate that the proposed accelerometer is suitable for high-g acceleration measurement applications</p>