The stiffness of microstructure is an important parameter of the microsensor. It is usually obtained by FEM approach, which is difficult to give an analytical expression. This paper reports on a 2-D microaccelerometer with the complex microstructure. Its main axial stiffness is carried out by using force method as well as principle of work and strain energy. During the deduction, deformation of all beams is taken into account, so the theoretical results are in accordance with that of FEM approach very well and the error is less than one in thousand. Furthermore, we neither introduce the other assumptions nor design the special dimension. So this methodology is strict in theory, and can be extended to analysis for the displacement and stiffness of the symmetric and statically indeterminate microstructure.