The activated charcoal moniter for thoron 220 Rn in air is applied into on the spot detection through analyse the method and the calculation of the data of detecting thoron 220 Rn in air.At the same time,the relevant method of settlement and the measure of improvement are introduced in accordance with disturbance which appeared when spectrum is surveyed,which in dicates that it is effective to use activated charcoal to collect and measure thoron 220 Rn in air.