The present invention discloses a method and device for detecting broken plastic strands of plastic extruder by heat. In this method, a detection strip is placed below the die exit of plastic extruder. The detection strip is made of a material that is the same as the product being produced by the plastic extruder or a material with similar melting performance as the product being produced. In the event of a broken strand during production, the broken strand will hang down and fall on said detection strip, where the residual heat in the strand can heat the detection strip to over its glass transition temperature and change it from glassy state to elastomeric state until it is pulled or molten to break. Once the break of the detection strip is detected, the alarm will be turned on. The detection device comprises a detection strip, a strip support, a contact detector(s) and a sound-light alarming device. One end of the strip support applies a pre-load upward or downward on the detection strip. The present invention is simple in structure, easy to configure to realize saved labor cost, improved product quality and reduced loss and waste. Thus, it has very promising prospects to be applied widely.