Off-axis integral cavity output spectroscopy technology has the characteristics of simple experimental setup, high sensitivity and fast response time, and is widely used in various ultra-sensitive gas detection fields. As an important part of the system, the high reflectivity mirrors are one of the important factors affecting the accuracy measurement of the entire spectral system. A set of off-axis integral cavity output spectral measurement system was built with the radio frequency noise source. Firstly, the absorption spectrum line of CH₄ gas at 6046.96 cm^-1 was taken as the research target, and the reflectivity of the cavity mirror was calibrated under different pressures with and without noise sources. The results show that the reflectivity calibrated under two different conditions, with and without noise sources, is consistent, and the reflectivity tends to decrease with the increase of pressure. The calculated highest reflectivity of the lens is about 0.99992. Furthermore, the CH₄ measurement signal at a concentration of 0.4 μmol/mol with and without noise sources was studied. It is found that after introducing noise sources, although the signal peak height is reduced, the signal-to-noise ratio is increased by about 1.3 times, and the minimum detectable concentration is 0.0045 μmol/mol, which indicates that the developed system can be effectively used for high sensitivity measurement of CH₄ in atmospheric environment and industrial applications.