The thermal target under complex background environment can be detected effectively by the technology of pyroelectric infrared detection. In this paper response characteristic of temperature difference for pyroelectric infrared sensor is investigated. Temperature difference between the target and background is correlated with effective incident radiation at the pyroelectric infrared sensor caused by the target and background. Here expansion target and small target are considered separately. The results show that response characteristic of infrared sensor is not in accord with linear relation. The higher background temperature causes higher response amplitude at the same temperature difference. Given dimension and temperature of the small target, the response amplitude of infrared sensor is decreasing as distance between the target and the sensor increasing.