The planar capacitance sensor is specially designed for the process of the test which can induce the changes of the water in the building envelope.The working principle of the sensor is given.In order to solve the interference problem of small capacitance in dectection

a sensor with protection electrode and the singal transmission technology of actuation electric cable are used.With the improved micro-capacitance measurement circuit based on charging/discharging principle

the optimization is given to the relations of test depth and sensitivity of the sensor.The system of moisture test in the building envelope is based on embedded microprocessor unit.The experimental results show that the system can be used for moisture test of the building envelope in a certain depth at the scene.