Off-axis integral cavity output spectroscopy (OA-ICOS) has the advantages of high sensitivity and strong environmental robustness. After the re-injection method is introduced, the signal power of OA-ICOS is effectively improved. It alleviates the problem that the detection sensitivity is highly dependent on the power of light source, and makes it possible to use low-power laser and high-reflectance cavity mirror to build the high sensitivity OA-ICOS. However, due to the special optical structure of the method, the detection signals tend to be doped with relatively strong interference noise. For interference problem, a re-injection OA-ICOS device is designed and built to measure the actual atmosphere CO2. The noise introduced by the re-injection structure is studied, and disturbance method is proposed to decrease the noise. Results show that the disturbance method reduces the amplitude of noise signal from the original 467.3 mV to 61.2 mV, which is 7.63 times lower. The method has the advantages of lower cost and easier implementation.