Abstract: The SD-3A Auto-radon detector is a kind of digital,sensible and stable instrument which can automaticaly measure gas radon under various conditions.The scintillant chamber is renovated and some new techniques are used to the ZnS (Ag)screen,which makes the instrument damp-proof and anti-erosion.The lifetime of the scintillometer has been extended and the application environment has been widened.The scintillomenter has a 51 series of microcomputer with a powerful 89c52-core.As soon as connected to the power and inputted necessary parameters,the instrument can run for a long time and reliable data can be obtained.The data can be transmitted when the instrument is connected to seismological web.The instrument,therefor,should be the first choice for measuring gaseous radon.