This paper performs a numerical investigation of a micro-electro-mechanical-system (MEMS) based capacitive viscometer. The viscometer includes a pre-stressed membrane interacted with a trapped air and a polymer solution, as well as a semi-permeable membrane that allows water and small molecular to pass through freely. A very simple modeling method is presented to solve the complicated problem. First, the fluid property of the polymer solution is estimated using relaxation time estimation. Second, a two modes vibration assumption of the pre-stressed membrane is proved using structure-air-interaction. Then the leakage through the semi-permeable membrane is also estimated using the volume flow with lubrication theory. Finally, the equivalent model of the glucose sensor is built using very simple vibration equations, the capacitance is also calculated. The simulation results show that the trapped air plays a significant role for the vibration of membrane, the volume should be large enough to reduce the nonlinear effects caused by the trapped air, which unfortunately causes the capacitance much to be smaller.