To study the transmittance stability of germanium-based infrared films in tropical marine environments, two samples: germanium-based double-sided DLC film and germanium-based double-sided high-efficiency antireflective film, were placed in a tropical marine environment for a natural exposure test under a shed, with a test cycle of six months and a total of four cycles. The transmission spectrum and microscopic morphology of the test samples from each cycle were analyzed. The variation pattern of the samples over time was examined through transmission spectroscopy results, and the stability and change mechanism of the infrared thin film transmittance were studied through microstructural analysis of the film layer. The test results show that the transmittance of germanium-based infrared films does not change significantly during one testing period under the shed. The overall waveform of the film remains unchanged with increasing test time, although the crest shifts toward longer wavelengths. After four test cycles, the transmittance of the high-efficiency antireflective coating was significantly reduced, whereas the transmittance of the DLC membrane did not decrease significantly, indicating good stability in a tropical marine environment.