Online sensors which are used to testing moisture play an important role in greenhouse system. Testing moisture which sensors is a key role to the accuracy is very important to agricultural development. The character of the sensors was directly associated with the development of greenhouse automatic system. There is no doubt that the structure of the sensors is critical to the testing results. A study of different sizes of sensors and the structure of capacitance sensors on precision was studied in the paper. We design the structure of the sensors and then conduct an experiment. According to the experiment, we decide the best size of the sensors.