Coking is a frequently happened phenomenon of heat transfer oil.The coking may cause blockage of pipe,decrease of heat transfer efficiency,even fire due to pipe burst.The physicochemical index and chemical composition of coking were introduced.It was pointed out that the coking of polypropylene extruder is due to the temperature of the oil film is obvious high the monitoring temperature and the flow velocity of heat transfer oil is too slow.The hazards caused by coking of heat transfer oil were analyzed,and the health state maintenance methods of heat transfer oil and relative equipments were discussed.