The present invention discloses a gas phase fluid bed polyethylene condensation technique dew point detecting method, which includes steps below: 1) An acoustic emission signal receiver is arranged in any position of a pipeline between a fluid bed heat exchanger outlet and a fluid bed reactor inlet for recirculating gas; 2) Acoustic emission signals are selected on a recirculating pipeline; 3) Frequency f, amplitude A and energy E of the acoustic emission signals and energy fraction R_i in each range of small wave or/and small wave packet. Moment of step transition for the energy fraction indicates temperature that unreacted gas flows are condensed into liquid flows, viz. the dew point. A vibratory receiving device of the present invention is not plugged and is conveniently and simply mounted. Vibration signals are generated during gas flowing, thus ensuring safety and environmental protection. In addition, present invention requires inferior measuring conditions, realizes around-the-clock operation under poor environment and has the advantages of sensitive response, accurate measuring, slight measuring error and wide applicable scope.