Many industrial blackbody furnaces are nonisothermal cavities. It is obvious that the calibrating errors of infrared(IR) thermometers are caused by those nonisothermal blackbody furnaces. In this paper, a narrow-waveband IR thermometer(Si-cell detector) is calibrated and researched by means of experiment and theoretical calculation. The law of the calibrating errors is found. According to the theory of the effective temperature of a blackbody cavity, the calibrating errors are calculated. The calculating results are basically consistent with the experiment results.