Abstract: A model of the capacitance ratio for a MEMS membrane switch is developed,which has considered the effect of threshold voltage,hold up voltage,bias voltage and the field strength in the dielectric layer on the capacitance ratio, thus it can effectively describe the characteristics of the capacitance for the switch. The capacitance ratio for a MEMS switch is numerically calculated,the results are analyzed and discussed. The method of using the pulse signal was proposed to decrease the thickness of the dielectric layer to about 50nm,and the capacitance ratio can be increased to 3800. Finally,the realization feasibility of the MEMS switch with high capacitance ratio is discussed.