The issues of absorbed-gas-induced frequency shift of a microcantilever with various geometries and profiles are investigated in this paper to enhance the measurement sensitivity of a microcantilever-based gas sensor. The resonant frequency of microcantilevers with different shapes is investigated by the analytical method and finite element method. Considering the dependence of frequency shift on the mass distribution of absorbed gas and structural resonant frequency, a principle is established for increasing the measurement sensitivity when the target gas is absorbed. Using these results, a stepped microcantilever with a trapezoidal-shaped part at the clamped end and a rectangular pad at the free end, which is expected to have better performance, is proposed as a sensing element in gas detection.