Based on the simultaneous temperature difference of the HDPE film defect area and the intact area, defects of HDPE film were detected with infrared thermal imaging technology. Under the action of a continuous heat source: i) infrared images are collected for defects of different areas and shapes, ii) the temperature of different areas of the film surface is recorded, iii) the temperature changes of different locations and the shadow area of infrared defects with time are analyzed, and iv) finally, the temperature characteristic curve and the best defect detection time limit were studied. The experimental results show that the temperature trends of the defect and complete area are the same the whole time; however, there is a synchronous temperature difference, and the infrared temperature image acquisition time is clearly in the 10-20 mins. It can be regarded as the best detection time domain of the defect. When the thermal image acquisition time is 13 mins, the infrared image edge contour is the same as the real defect contour; therefore, it is the optimal detection time point.