Abstract: An automatic radon measuring instrument type JSZ-2 for observing the continuous change of radon concentration of underground water in the search of earthquake precursors is described in this paper. It consists of four essential portions, namely, (1) continuous degassing device made of glass; (2) automatic programming controller of timing; (3) scintillation impulse counter; (4) digital repeated recorder.The set can be used to record intermittently or continuously for radon content of underground water, maximum 1, 000 Emans, with errors ±10%. The quantity of water flow Q_water=2.0 liters/minute and of gas flow Q_gas=200—450 milli-liters/minute. Sensitivity is about 800 impulses/minute/Eman., response, less than 5 minutes, efficiency of degasification about 50%.Under various conditions, this instrument has been subjected to tests for its actual performance. It can be said that in comparison with instruments of similar nature, made both in China and abroad, this instrument has the characters of better direct viewing, higher sensitivity and reliability, ease of operation, etc. Particularly, the glass degassing device has the advantage of anti-corrosion, ease of cleaning and low cost. The instrument can essentially meet the requirements of Radon observation in earthquake areas.