The invention provides a polyvinyl chloride synthesis process low-boiling column exhaust condensation on-line monitoring and alarm method. The method is characterized in that the changing trend of the temperature is estimated by employing the temperature of an outlet of an exhaust condenser, and the decrease of the condensation efficiency is detected as soon as possible by employing derivative information. If the efficiency of the exhaust condenser is reduced and the phenomenon of freezing occurs, the temperature of the exhaust outlet will be obviously reduced.