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Although thermal infrared (TIR) imaging technology has been successfully applied in the thermographic inspection of engineering defects and landmines, its use for detecting water permeation on field large-scale rock relics is new. Huashan Rock Painting, a world-famous field large-scale rock relic in Guangxi Province, China, was used as a case for TIR imaging detection of water permeation on field large-scale rock relics. This paper introduced detection methods, a key parameter, and a gradation model for TIR imaging. Three detection methods were specially designed and used in two experiments performed during the drought and rainy seasons, respectively. Thermographic features were comprehensively analyzed, and temperature variation curves of water-permeation places were quantitatively compared with those of places unaffected by water permeation. Water-permeation places can be precisely identified based on thermographic features and temperature variation curves. Physical interpretation was conducted based on the heat transfer equation and on the Stefan-Boltzmann law. The minimum temperature of solar irradiated water-permeation places was selected as the key parameter, and a -based model was established for water-permeation gradation. To strengthen the differences between water-permeation places and places unaffected by water permeation, TIR imaging should also be conducted during a sunny day after the rains, particularly during an intermittent sunny day after heavy rains in the rainy season. It is the best time for TIR-imaging-based water-permeation detection and gradation.