This paper put forward a method for analyzing effect of Earth Infrared (IR) radiation on exoatmospheric temperature measurement accuracy. Based on principle of 2-waveband IR temperature measurement, the calculation formula for color-temperature measured by IR sensors is derived and analysis method of measurement error is given when taking into account Earth IR radiation reflected by measurement targets. Then taking Ballistic Missile Defense (BMD) system EKV IR sensor’s temperature measurement on strategic ballistic targets as an example, the scale of effects on temperature measurement accuracy and discrimination capability of the sensor were analyzed when taking into account the Earth IR radiation by applying Monte Carlo method. Based on analysis above, it is concluded that the temperature measurement accuracy of the IR sensor on exoatmospheric objects is obviously degraded by Earth IR radiation.