Abstract: It is the inevitable trend of developing the observation methods of fault soil gas in earthquake fluid observation when the water resources is under the situation of extra-mining.The observation of Rn-content is always the importantly item in earthquake fluid observation measures.But the underground fluid observation technology standards has not given the concrete parameter of gathering-collecting gas device of Rn-observation in fault soil gas.These parameters are key factors to affect the observation result.Therefore, This article has done the experiment on different observation device and collecting conditions.The result indicated that, the gathering chamber detention volume have affect to observe result greatly, but the dead volume not.Therefore, the gathering chamber effective size should not retain the detention volume.Use the concept "collecting gas rate" to judge the practical effect of the fault soil gas observation system quantitatively.The gathering chamber effective size should act according to amount of the amount of collecting gas.The more the amount, the larger the gathering chamber, the "collecting gas rate" should be equal to 1.This article experimental result has provided the scientific basis to formulates the observation technology standards of Rn-content in the fault soil gas.