Abstract: On the basis of theory of plasma sheath,the paper expounds the mechanism that plate or probe has negative potential when detecting plasma efflux or plasma cloud.If the probe is located in the plasma,there is a negative electromotive force in the probe.It offers the formula to compute plasma cloud voltage.The value has only something to do with temperature and plasma component.A device of detecting plasma cloud without power supply is designed.The keyhole is formed if the sheath voltage is zero.The experiment proves the theory of the sheath.At the same time,the paper gives the right location of probe to detect the plasma cloud.