Off-axis integrated cavity output spectroscopy (OA-ICOS) is a highly sensitive laser spectroscopy technique. However, due to the use of dense high-order modes for detection, OA-ICOS signal power is low, thus making the detection sensitivity highly dependent on the laser power. To this problem, we introduce an optical re-injection method to re-inject the laser back into the optical cavity again, improving the utilization of laser energy and the power of signal. In this paper, we use optical tracking software to design a re-injection structure, and study several factors affecting the signal gain. Then, we build a re-injection OA-ICOS device in the 2 μm band and also conduct a series of experimental researches. Our results show that the re-injection method enhances the OA-ICOS signal power 8 times and signal-to-noise ratio 4.6 times, which effectively improves the detection sensitivity and the absorption depth of the spectral signal, and alleviates the problem of low signal power in OA-ICOS detection.