Defects of transmission lines are less harmful than faults, but the defects’ frequency is much higher than the faults’, and defects can change to serious faults when their amount increases. In order to protect the normal running of transmission line, it needs to take precautions against the defects. Based on the classical association analysis method Apriori, an improved method is proposed, which could find out key association factors of defects by combining the meteorological external force, equipment, terrains and defects in the spatial-temporal viewpoint. Taking Xinjiang 750 kV UHV transmission line as an example, it is found that the defects of 750 kV UHV transmission lines in Xinjiang show the following characteristics from year 2012 to year 2015:strong wind is the main external meteorological factor of defects, and their internal factors are the damage and loss of steady pins. In the spatial aspect, the latitude (N 43~43.5 degree), aspect (158~202 degree) and towers’ height (50~55 meters) are strongly associated with defects. Especially, in meteorological aspect, in 24×7 hours the wind direction keeping in the 175 to 235 degree is strongly associated with defects. This work is supported by Natural Science Foundation of Hubei Province (No. 2014CFB194).