Abstract：Body fluid is an important substance that constitutes and maintains human life activities，the key to the transmission and change of cold damage，and also an important method for the application of classic formulas in syndrome differentiation and treatment. Preserving body fluids is an important academic idea in the syndrome differentiation system of Shang Han Lun (Treatise on Cold Damage). ZHANG Zhongjing used the methods of sweat promotion， purgation， harmonization， and supplementation in the differentiation and treatment of taiyang disease， mingyang disease， shaoyang disease， and three yin disease， which contain the concepts of preserving stomach fluid， timely purgation to preserve yin， unclogging and regulating fluid，and restoring body fluid，and these concepts run through prescription and medication，post-medication care，decoction methods，and other treatment processes. From the aspect of body fluids，the authors interpret the rules of movement and changes of body fluids in the six-meridian syndrome differentiation in Shang Han Lun (Treatise on Cold Damage) and sort out the clinical application of the idea of preserving body fluids in the six-meridian syndrome differentiation，to provide better guidance on clinical practice.