Abstract: The open loop testing is a necessary step in the studying of silicon resonant pressure micro sensors. The paper introduces a special open loop testing system designed by the authors. This testing system is an automatic one that can give the amplitude frequency and phase frequency properties of the resonant beam. The weak signal, about several micro volts, given by the sensing resistor can also be detected successfully by this system. As a most important problem, the so called common frequency disturbance (CFD) has been discussed. It is found that the exciting signal coupled through the distributed capacitor between the exciting resistor and the sensing resistor is the final source of CFD. A sympathetically exciting technique is used to solve this problem. It works well.