Analysing thermal infrared anomaly images before earthquakes more than 40 in China and its neighboring areas, the main features of the infrared anomalies are summarized.In general, there exist clearer anomalies with a bigger amplitude of temperature increase which show the character of suddenness and phase before M_S >5 earthquakes.Abnormal area is related positively to magnitude with a certain degree.Some studies on mechanism of the infrared anomaly are introduced and the author's trend opinion is put for ward.It is believed that the polarizing function of air static electric field anomaly to the atmosphere impending before earthquake should be the major factor to the infrared anomaly.And "the greenhouse effect" of the atmosphere containning abundant "polluting air" should be another factor that can not be ignored.