A High-dynamic-range infrared (IR) radiometer is essential for calibrating a wide-temperature-range IR radiation source. However, a low-temperature radiation source has very weak IR radiation which is easily submerged in noise. In this paper, a chaos detection method based on Diffing-Holmes oscillator (DHO) was adopted to enhance the ability of the IR radiometer for measuring weak IR radiation. The power measurement range of the radiometer was 2.9 × 10-9-5.9 × 10-2 W. The minimum SNR that was detected by the radiometer was as low as -22.46 dB. The noise during the measurement including the mismatch noise, the stray noise, and the detector noise were analyzed. The radiometer is calibrated by measuring a radiation source with adjustable temperature and aperture. The results show that the responsivity of the radiometer is 8.61257 × 104 V/W. The combined standard uncertainty of the radiometer is less than 1.66%. The IR radiometer introduced in this paper is capable of calibrating the IR radiation source of ultralow temperature, and has been applied in the ground test of a space-borne IR detection system.