Abstract: The ion concentration in the air is closely correlated with the environment.The variation rules of negative air ion concentration with the change of temperature,humidity or simultaneous change of temperature and humidity were studied under natural conditions.It was found that the negative ion concentration rose quickly with the increasing relative humidity(RH) and it could change from 200 ions/cm~3 to above 8 000 ions/cm~3 when humidity gradually increased(RH:10%～80%).The negative ion concentration also went up with temperature rising(5～40 ℃).The variations of concentration were even greater with simultaneous increase of temperature and humidity.